AMER. ZOOL., 25:73-85 (1985)
Putative Neuroendrocrine Devices in the Nemertina—
An Overview of Structure and Function1
JOAN D. FERRARIS
Mount Desert Island Biological Laboratory, Salsbury Cove, Maine 04672
SYNOPSIS. In the Nemertina there are three putative neuroendocrine devices: the cephalic
gland, the cerebral organs and the neurosecretory system (cerebral ganglia). Cytological,
behavioral and physiological evidence suggests involvement of these systems with preydetection, reproduction and volume regulation. The neuroendocrine function(s) of each
of these devices must be considered putative except where classical ablation-replacement
experiments have been performed. Thus, a specific role in gonad maturation has been
successfully demonstrated for the cerebral ganglia, a primary neurosecretory center.
Although the evidence is not as strong, cytological and physiological studies indicate that
all three neuroendocrine devices, where they are present, are involved in response to
variation in environmental osmolality. In the physiological response to osmotic variation,
evidence most strongly points to neuroendocrine control (first order or second order) of
extracellular volume. In this control, two different mechanisms may be involved, i.e.,
stimulation of nephridial elimination of extracellular water and solutes and a change in
permeability of the integument or gut to ions. In first order reactions, the neuroendocrine
devices would act directly and independently upon a target organ such as the nephridia,
integument, or gut to regulate extracellular volume. In a second order reaction, neurosecretory material would stimulate enhanced release of acid mucopolysaccharides from
the cephalic gland or cerebral organs (depending on the species) into the vascular system.
These substances (active principle or carrier) would, in turn, affect regulation of extracellular volume via the nephridia, integument, or gut. Further experimentation is required
to separate the effects observed after neuroendocrine manipulation as well as the contributions made by each of the potential neuroendocrine devices.
ralis (Cephalothricidae: Archinemertini
after Iwata, 1960) is the only species in
which the gland cells are known to disCephalic glands
charge substances into the vascular system
The cephalic gland is present in virtually and is thus the only species, to date, to
all nemertines as a mass or masses of gland which a neuroendocrine function for the
cells that occupy much of the precerebral cephalic gland may be ascribed. In this
region (Fig. 1A-C). Historically, reference species, the cephalic gland occupies the
to the cephalic glands in the scientific lit- entire precerebral region and is deeply
erature has been primarily of taxonomic incised by narrow extensions of the cephalic
importance and is largely restricted to blood lacuna. In most other species, the
description of the size or position of these association of the cephalic gland with the
structures in relation to other organs (Pan- vascular system is less intimate, because of
tin, 1969; Moore and Gibson, 1972). The intervening layers of parenchyma, and
cephalic glands are associated with the ner- cephalic gland cells discharge their secrevous and vascular systems to varying tions to the exterior of the animal.
degrees depending on the species. These
In P. spiralis, the structure of the cephalic
associations are of particular importance
in ascribing function to the cephalic glands gland can be considered neuroglandular.
since these structures may not be analagous A cephalic nerve from each of the cerebral
in the species that possess them (Ferraris, ganglia extends along the cephalic blood
1979a). In this regard, Procephalothrix spi- lacuna into the cephalic gland (Fig. 1 A) and
numerous Type A neurons (Ferraris,
1979a) are scattered among the cephalic
gland
cells. More commonly, the cephalic
1
From the Symposium on Comparative Biology of gland does not share such an intimate assoNemertines presented at the Annual Meeting of the ciation with the nervous system (Fig. IB,
American Society of Zoologists, 27-30 December
C).
1983, at Philadelphia, Pennsylvania.
STRUCTURAL ASPECTS AND HISTORICAL
PERSPECTIVE
73
74
JOAN D. FERRARIS
tine gland cells made by Ferraris (1979a).
However, no known differentiation of
function among the various gland cell types
has been established yet.
Cerebral organs
FIG. 1. Semidiagrammatic representation of the
internal morphology of the anterior region of A) Procephalothrix spiralis, B) Lineus soaalis, and C) Amphi-
porus lactifloreus. View is dorsal. Scales equal 250, 300
and 450 ^tm, respectively. Abbreviations: CC, cephalic
canal; CG, cephalic gland; CL, cephalic blood lacuna;
CN, cephalic nerve; CO, cerebral organ; CON, cerebral organ nerve; CS, cephalic slit; DG, dorsal ganglion; FO, frontal organ; LBV, lateral blood vessel;
P, proboscis; RD, rhynchodaeum; RH, rhynchocoel;
TZ, transition zone. Reproduced with permission from
Ferraris (1979a).
T h e gland cells that comprise the
cephalic gland have been studied, in detail,
in only a few species. The gland cells have
been placed in the general categories of
acid mucous cells and serous cells. Serous
cells contain proteinaceous secretions
(Gontcharoff and Lechenault, 1966).
Characteristics of the cell types found in
three species in the Archi-, Hetero- and
Hoplonemertini have been described and
comparison with other studies on nemer-
Cerebral organs are neuroglandular
structures that are characteristic of nemertines. The few species in which they are
absent or degenerate normally occupy relatively constant or specialized habitats; i.e.,
the Polystilifera (Hoplonemertini) that lack
them are bathypelagic while Carcinonemertes (Hoplonemertini) and Malacobdella
(Bdellonemertini) are ectocommensal.
However, the Cephalothricidae lack cerebral organs but are free-living and intertidal in habit.
The cerebral organs historically, have
been considered to be sensory in function
due to their intimate association with the
nervous system and the presence of a ciliated canal (cephalic canal) that is continuous with the external environment (Fig.
2A-C). Quatrefages (1846) originally
thought them to be an auditory apparatus.
However, Dewoletsky (1887) and Burger
(1897-1907) thought the cerebral organs
to be chemosensors for determining the
conditions of water, and Reisinger (1926)
a sensor for food detection. Other functions such as excretion and respiration were
suggested by Mclntosh (1873-1874) and
Hubrecht (1875), respectively. While a
sensory function is not disputed, the intimate association of gland cells with both
neural tissue and the vascular system suggested some additional, perhaps neuroendocrine, function (Scharrer, 1941; Ling,
1969a, 1970; Willmer, 1970).
The detailed structure of the cerebral
organs, which in many instances is highly
complex, is known for only a few species.
They are, among the Heteronemertini,
Lineus ruber (Ling, 1969a, b, 1970), L.
socialis (Ferraris, 19796) and Cerebratulus
marginatus (Bianchi etal., 1972); and among
the Hoplonemertini Amphiporus lactifloreus
(Ling, 19696; Ferraris, 19796), Paranemertes peregrina (Amerongen and Chia,
1983) and Tetrastemma (Amerongen, in
preparation, cited in Amerongen and Chia,
1982). There is great similarity and con-
NEUROENDOCRINE DEVICES IN THE NEMERTINA
75
FIG. 2. Semidiagrammatic representation of A) the cerebral organ of Lineus socialis in sagittal section, B) in
cross section through the plane of X-X' of A, and C) of Amphiporus lactifloreus in sagittal section. Adapted
from Ling (1969a, 1970). Scale equals 50 jjm. Abbreviations: BC, basal cell; BP, bipolar cells; CC, cephalic
canal; CI, ciliated cells; CL, cephalic blood lacuna; CON, cerebral organ nerve; CS, ciliary septum; CT,
connective tissue; DG, dorsal ganglion; GA, GB, GC, gland cells Type 1 group A, B, C, respectively; GG,
ganglion cells; LBV, lateral blood vessel; MI, minor canal; MJ, major canal; SP, secretory pool; TZ, transition
zone; VCL, vesicular cells (Type 2 gland cells); VE, vascular epithelium. Arrow indicates anterior in A and
C. Reproduced with permission from Ferraris (19796).
sistency in the basic structural organization
and composition of the cerebral organs in
all of these species (although terminology
differs among authors). Thus, a single
description will suffice for both taxonomic
groups with differences noted where they
apply.
Cerebral organs are paired structures
largely composed of nerve and gland cells.
In heteronemertines, each is connected or,
in some cases, fused to the posterior region
of the dorsal cerebral ganglion (Fig. IB).
Cerebral organs are anterior to the ganglia
in hoplonemertines and are connected to
the corresponding dorsal ganglion by a
cerebral organ nerve (Fig. 1C). Cerebral
organs are in contact with the external
medium via a ciliated tube (cephalic canal)
that originates as an inward extension of a
cephalic slit or groove (Fig. 2A-C). The
cephalic canal, making various bends along
the way, passes through the nervous and
glandular tissue and terminates blindly at
or near the vesicular tissue (Type 2 gland
cells) in the posterior region of the cerebral
organ. Most of the cephalic canal is lined
by ciliated cells, between which processes
of bipolar nerve cells extend to the lumen
of the canal. Thus, the external medium
that enters the canal comes in contact with
a sensory epithelium. In lineids, the
cephalic canal is divided longitudinally into
a major and minor canal by a ciliary septum
(Fig. 2A). Cilia lining the canal are differentiated into long cilia in the major canal
where the effective stroke is inward and
comparatively short cilia in the minor canal
where the effective stroke is outward (Ling,
1969a). A similar division of the canal is
observed in P. peregrina (Amerongen and
Chia, 1983) whereas the division in A. lactifloreus is only partial (Fig. 2C). However,
in the latter, the cilia on medial and lateral
halves of the canal are differentiated as are
those in the lineid major and minor canals,
respectively (Ling, 19696). This regional
76
JOAN D. FERRARIS
specialization may indicate similar functional significance in direction of effective
stroke.
Various cell types comprise the cerebral
organs. For a detailed description, based
on electron microscopic and histochemical
evidence, see Ling (1969a). Bipolar nerve
cells make up a large portion of the organ
and possess a high level of cholinesterase
activity (Ling, 1969a). Dendrites extend
from these cells to ramify between the ciliated cells of the cephalic canal. Their axons
intertwine with each other to form the neuropil of the cerebral organ nerve (Ling,
1969a). In lineids, Type A ganglion cells
(Ferraris, 1978, 19796) occur among the
bipolar cells in the transition zone, which
lies between the cerebral organ proper and
the cerebral ganglion. Based on their selective staining affinity, some ganglion cells
(Lechenault, 1962, 1963; Servettaz and
Gontcharoff, 1973; Ferraris, 1978), and
possibly also some bipolar cells (Ling,
1969a), are considered neurosecretory.
Two types of gland cells are found in the
cerebral organs (Fig. 2A-C). Mechanisms
of secretion were studied by inducing a
secretory cycle in the cerebral organs of L.
its terminus within the organ. Basal regions
of vesicular cells either abut upon the connective tissue sheath surrounding the
organ, as in lineids (Fig. 2A); or extend
posteriorly among the basal cells (see below)
which form the "secretory pool" (Ling,
19696) of the organ, as in A. lactifloreus
(Fig. 2C). Type 2 gland cells contain precursor globules and comparatively smaller
secretory vesicles, both of which appear
intravacuolar. Precursor globules containing neutral mucopolysaccharide phagocytized from the canal are situated apically.
Here, it should be noted that Bianchi et al.
(1972) report no relationship between the
secretion of Type 1 and Type 2 gland cells.
This discrepancy may reside in the fact that
a secretory cycle was not experimentally
induced in their material. As the precursor
globules move basally they are modified by
fusion with intracellular microvesicles. The
amount of carboxyl and sulfhydryl groups
in the precusor gobules increases until the
final acid mucopolysaccharide of the secretory product is formed.
Cerebral organs contain yet another cell
type, the basal cell. Basal cells are spindleshaped or irregular in outline and are filled
ruber, L. socialis and A. lactifloreus by sub- with secretory vesicles indistinguishable
jecting the worms to hypoosmotic condi- from the secretory vesicles that occur in
tions. These experiments, in combination basal regions of vesicular cells. Basal cells
with light and electron microscopy and his- obtain the secretory vesicles by phagocytochemical procedures, led to the formu- tosis and are located between the Type 2
lation of a mechanism of secretion for the gland cells and the connective tissue that
cerebral organs (Ling, 1969a, b, 1970; Fer- surrounds the organ. In A. lactifloreus they
raris, 19796). That mechanism is incor- form an extensive secretory pool. A similar
porated into the following description to pool exists in P. peregrina (termed Type 2
facilitate explanation of the interrelation- vesicular material [Amerongen and Chia,
1983]).
ship between cell types.
Lastly, and very important in ascribing
Type 1 gland cells contain neutral mucopolysaccharide and open into specific function to the cerebral organs, is their
regions of the cephalic canal where the relation to the vascular system. In lineids,
effective stroke is directed inward (Fig. 2B, the cerebral organ lies within a lateral blood
C). Neutral mucopolysaccharide is dis- vessel and secretion into the vascular syscharged from these cells, by exocytosis, into tem occurs by diapedesis of basal cells. Basal
the lumen of the cephalic canal where it is cells, loaded with secretory vesicles, migrate
transported to the Type 2 gland cells by through the connective tissue sheath of the
ciliary action. Type 2 gland cells (vesicular organ and the vascular epithelium of the
cells) are large and have ill-defined bound- blood vessel. In the hoplonemertines A.
aries. They appear "foamy" due to the lactifloreus and P. peregrina, the cerebral
presence of countless intracellular micro- organs are also in close association with the
vesicles (Ling, 1969a). The apical necks of vascular system; one arm of the cephalic
these cells line the cephalic canal at or near blood lacuna runs alongside each cerebral
NEUROENDOCRINE DEVICES IN THE NEMERTINA
77
organ. However, in contrast to the situation in lineids, secretion into the vascular
system in A. lactifloreus apparently occurs
by release of secretory vesicles from basal
cells. Vesicles, indistinguishable from the
secretory vesicles in basal cells, are found
in the tissue between the cerebral organ
and the blood vessel. In Amphiporus such
vesicles are easily distinguished, by virtue
of staining affinity and great difference in
size, from the large acid mucopolysaccharide-containing "vacuoles" (Klappenzellen
after Bohmig, 1898) present in the wall of
the blood vessels. This may not be the case
in all species, since similarity in size and
staining affinity is noted in Paranemertes
(Amerongen and Chia, 1983). However,
secretion into the vascular system has not
been observed in this species. The secretory vesicles of both lineids and A. lactifloreus may lose their sulfhydryl groups after
release from the cerebral organs. This corresponds temporally with the appearance
of sulfhydryl-containing droplets in the
vascular system and rhynchocoel (Ferraris,
19796). Here it should be noted that it is
thought that there is communication
between the vascular system and the rhynchocoel via rhynchocoel villi or vascular FIG. 3. Semidiagrammatic representation of the right
plugs.
dorsal and ventral ganglia of A) Procephalothrix spiralis, B) Lineus socialis, and C) Amphiporus lactifloreus.
Neurosecretory system
The cerebral ganglia and the lateral
nerve cords are primary neurosecretory cell
centers in nemertines. The morphology of
the cerebral ganglia in most species follows
the basic nemertine plan with some morphological adaptation to the presence and
position of the cerebral organs (Fig. 1AC). In general, the ganglia and major nerves
consist of an inner core of nerve fibers
(neuropil) and outer layers of perikarya.
Briefly, there are four lobes comprising the
cerebral ganglia, one pair dorsal and one
pair ventral. The ganglionic lobes are partially fused to each other dorso-ventrally and are also united dorsally and ventrally by a commissure. Thus, they encircle
the rhynchocoel and the cephalic portion
of the vascular system. Posteriorly each
ganglionic lobe is separate. The ventral
lobes continue posteriorly as bilateral nerve
cords (Fig. 3A-C). Several nerves arise from
View is dorsal. Scales equal 50, 90, and 90 nm, respectively. Abbreviations: CC, cephalic canal; CN, cephalic
nerve; CO, cerebral organ; CON, cerebral organ
nerve; DC, dorsal commissure; DGN, dorsal ganglion
neuropil; DN, middorsal nerve; DPT, dorsal posterior trunk; EN, esophageal nerve; LN, lateral nerve
cord; VC, ventral commissure; VGN, ventral ganglion neuropil. (All other abbreviations refer to zones
of the central nervous system as in Ferraris, 1978,
Table 1.) Reproduced with permission from Ferraris
(1978).
the cerebral ganglia; large nerves connect
the dorsal ganglia with the cerebral organs
while cephalic nerves extend anteriorly
from each ganglionic lobe.
The study of neurosecretion in the Nemertina has a comparatively short history.
Investigators have employed such histological techniques as chrome-hematoxylinphloxine, paraldehyde fuchsin and pseudoisocyanine to identify neurosecretory cells
(NSC) or neurosecretory material (NSM)
in various Hetero- and Hoplonemertini
78
JOAN D. FERRARIS
TABLE 1. Correlation among described neurosecretory and ganglion cell types.
Canglion cells
Burger, 1897-1907
Montgomery, 1897
I
II
III
Neurosecretory cells
Lechenault, 1962
1
2
Bianchi, 1969a
a(al,a2)
b
c(cl,c2)
d
Servettaz and Gontcharoff,
1973 and 1974
Ferraris, 1978
A (a, j3, y, i)
B
A
B
C
C
Reproduced with permission from Ferraris (1978).
(Lechenauk, 1962, 1963; Bianchi, 1969a,
b; Servettaz and Gontcharoff, 1973, 1974,
19766). For a more detailed discussion of
the pertinent literature see Ferraris, 1978.
Based on morphology and histochemistry,
three to four neurosecretory cell types have
been discerned (Table 1). Cell types are
subdivided based primarily on position in
the nervous system. There is general
agreement that the neurosecretory cells
and the NSM in the neuropil contain glycoproteins that are rich in sulfated and basic
amino acids (Lechenauk, 1963; Servettaz
and Gontcharoff, 1973, 1974, 19766).
Bianchi (19696), however, does not believe
that carbohydrates are present in all cell
types. The NSC of three species of nemertines in the Archi-, Hetero- and Hoplonemertini were also examined utilizing
Alcian Blue/Alcian Yellow (Ferraris,
1978). Three cell types were discerned and
each zone of the cerebral ganglia mapped
with respect to distribution and abundance
of each type (Fig. 3A-C; Ferraris, 1978,
Table 1). Since there is no known functional differentiation among cell types, no
consistent difference in histochemistry, and
no difficulty in comparing the cell types
among the species, correlation is made,
purely on morphological grounds, with all
other NSC types described in the Nemertina as well as classical studies performed
on nemertine ganglion cell structure
(Burger, 1897-1907; Montgomery, 1897)
(Table 1). The neurosecretory cells of all
three species, regardless of type demonstrate variable staining affinity to AB/AY;
which is interpreted as progressive stages
in the elaboration of the final secretory
product or NSM (Ferraris, 1978). Since
neurosecretory material within the neuropil usually demonstrates a high propor-
tion of sulfhydryl groups, the composition
of the presumed storage or final form of
NSM is thought to be similar for all cells.
Electron microscopic evidence of neurosecretory cells in the Nemertina is limited, but a few studies confirm the presence
of characteristic intracellular inclusions.
Thus, Ling (1969a)onotes electron dense
granules 200-1000 A in diameter in bipolar cells of the cerebral organs, which may
indicate aminergic secretion. However,
Reutter (1969) was unable to demonstrate,
histochemically, the presence of aminergic
fibers in the cerebral organs. Peptidergic
secretion, as evidenced by the presence of
electron dense granules 800-1200 A in
diameter (Servettaz and Gontcharoff,
1976a), occurs in the ganglion cells of the
cerebral organs.
Since elements of the vascular system lie
between the cerebral ganglia and the rhynchocoel (Fig. 1A-C), only aflattenedvascular epithelium separates the vascular fluid
from certain areas of the cerebral ganglia.
Thus, some regions of the cerebral ganglia
may be neurohemal or storage areas for
neurosecretory material prior to its release
into the body fluids. In heteronemertines,
Bianchi (1969a) suggests a primitive neurohemal area in the dorsal ganglia, while
Servettaz and Gontcharoff (1976a) suggest
the presence of one in the ventral commissure based on electron micrographs of
nerve fibers. In the Hoplonemertini, there
is ultrastructural evidence for terminals of
axons from neurosecretory cells in the portion of the rhynchocoel wall underlying the
dorsal commissure (Bierne, personal communication).
However, it is not necessary for NSM to
be released from axon terminals directly
into the vascular system in order to have
NEUROENDOCRINE DEVICES IN THE NEMERTINA
79
cephalic gland cells in P. spiralis and cerebral organ gland cells in L. socialis and A.
lactifloreus do not release acid mucosubstances into the vascular system. The
appearance of selectively stained droplets
in the body fluids also does not occur.
Under these conditions, accumulation of
secretory material in the gland cells occurs;
this may be the result of continued synthetic activity during a period in which
release is blocked (Ferraris, 19796, d).
Ling (19696, 1970) provides extensive
ultrastructural and histochemical evidence
for the same response in the cerebral organs
of L. ruber and A. lactifloreus under hypoand hyperosmotic conditions. More
recently, Varndell (1981) also corroborated release of acid mucopolysaccharides
into the vascular system when the same
species were subjected to hypoosmotic
media. However, when Amerongen and
Chia (1983) subjected Paranemertes peregrina to diluted seawater they attributed
the observed, similar cytological changes
FUNCTIONAL ASPECTS
in the cerebral organ gland cells to the
Cephalic glands and cerebral organs
passive effects of an osmotic gradient. This
Much of the evidence for a given func- was based on the occurrence of similar
tion in these putative neuroendocrine cytological events in animals that were not
structures is cytological. Thus the cephalic exposed to a hypoosmotic medium but had
gland of P. spiralis and the cerebral organs been fixed in a hypoosmotic fixative. From
of L. socialis and A. lactifloreus undergo a their results on the cytology of the gland
secretory cycle and apparently release acid cells it is not possible to separate the effects
mucosubstances into the vascular system of hypoosmotic fixation from exposure to
when worms are subjected to hypoosmotic a hypoosmotic medium since the same
conditions (Ferraris, 19796, c, d). Response hypoosmotic fixative was used in both cases.
is noted within 0.5 hr of the onset of osmotic They also did not observe secretion into
perturbation. In all species, droplets con- the vascular system. This differs from
taining acid mucosubstances are subse- observations made on other species and
quently observed in the vascular system or may indicate that the cerebral organs of
rhynchocoel. Here it should be noted that this species respond differently to hypoosP. spiralis lacks cerebral organs; thus the motic media, at least at the cytological level.
cephalic glands of this species may be analThe cephalic glands and the cerebral
ogous to the cerebral organs of other organs do not demonstrate diel rhythmicspecies. The cephalic glands of L. socialis ity in secretory cycle, nor is there an apparand A. lactifloreus similarly undergo a secre- ent relationship with sexual reproduction
tory cycle under hypoosmotic conditions (Ferraris, 1979a, b). Gontcharoff and
but, in these species, secretion is to the Lechenault (1958) originally suggested that
exterior of the worm (Ferraris, 1979d). In the cerebral organs may be the source of
this case, enhanced mucus production may a gonad inhibiting factor; however, selecserve as a protective device but the cephalic tive ablation and grafting studies by Bierne
glands cannot be considered neuroendo- (1966) on species of Lineus do not support
crine in nature. When the same species are this hypothesis.
immersed in a hyperosmotic medium,
In general, changes in conditions other
effective transport of NSM to receptor cells.
NSM may be released via synaptoids, which
resemble standard presynaptic specializations, morphologically, but are not restricted to the terminal region of the axon
(Scharrer and Weitzman, 1970). In this
case, release may be into a zone of intercellular stroma which serves as a link
between the NSC and the vascular system
or nearby effector cells. Similarly, neurosecretory cells may form a "neurosecretory neuropil" from which NSM diffuses
across tissue spaces to control adjacent
receptors. Release may also be via "neurosecretomotor junctions" whereby neurosecretory cells make direct contact with
effector cells (reviews: Bern, 1966; Scharrer, 1972). In any case, the neurosecretory
cell occupies a unique position whereby it
receives messages in "neural" language and
transmits information in modified "endocrine" language (Scharrer, 1967).
80
JOAN D. FERRARIS
than osmolality of the medium, such as elevated temperature, caudal amputation and
desiccation, do not consistently affect the
cytology of the cephalic glands or the cerebral organs (Ferraris, 19796, d). There is,
however, evidence for an influence of light
on the secretory cycle of the cerebral organs
(Ling, 1970) but not the cephalic glands
(Ferraris, 1979d). Under conditions of
increased light intensity, gland cells in the
cerebral organ of L. ruber demonstrate a
secretory cycle similar to that observed
under hypoosmotic conditions, whereas
"dark adapted" worms demonstrate the
reverse. Willmer (1970) also made similar
observations.
Neurosecretory system
In the neurosecretory system, there is no
evidence for a diel rhythm or any apparent
cytological response to increased temperature, variation in light conditions, or desiccation (Ferraris, 1978, 1979c). However,
there is evidence for involvement of the
neurosecretory system in sexual and also
perhaps asexual reproduction or regeneration. Thus, in P. spiralis and A. lactifloreus the NSC in the cerebral ganglia of
sexual animals (bearing ripe gonads) have
a greater affinity for neurosecretory stains
than do those of non-sexual adult worms
(Ferraris, 1978). This is interpreted as
higher secretory activity in the NSC of nonsexual worms and corroborates Bierne's
(1966, 1970) evidence (see below) for the
presence of a gonad inhibiting factor in the
cerebral ganglia. Servettaz and Gontcharoff (19766) suggest that Type B neurosecretory cells in species of Lineus may
be the source of the factor but work by
Ferraris (1978) does not indicate such a
specific role for the presumably corresponding Type B cells of"L. socialis. In terms
of asexual reproduction or regeneration,
evidence is limited but points to a difference in response to caudal amputation
depending on whether a given species is
capable of anterior, as well as posterior,
regeneration. L. socialis, but not P. spiralis
or A. lactifloreus, possesses these capabilities
and is also reported to undergo asexual
reproduction by fragmentation when nonsexual (Coe, 1930). Thus, when these
species are subjected to caudal amputation
when nonsexual, only L. socialis demonstrates apparent release of NSM (Ferraris,
1979c).
Cytological evidence for a role in
response to osmotic perturbation is also
found in the neurosecretory system of
nemertines. In three species, P. spiralis, L.
socialis and A. lactifloreus, there is apparent
release of NSM from neurosecretory cells
under hypoosmotic conditions but accumulation of the secretory product when
the osmolality of the ambient seawater is
increased (Ferraris, 1979c). All three neurosecretory cell types (A, B, C) respond in
a similar manner and do so within 0.5 hr
of the onset of the osmolality change. Thus,
there is no apparent functional distinction
discernable among the cell types under
these conditions. Lechenault (1965a, b),
working with L. ruber and L. viridis did,
however, report a temporal difference in
response between the large (Type C ?) and
small neurons (Types A and B ?) when the
worms were in diluted seawater.
Since all three putative neuroendocrine
devices in the Nemertina are responsive,
on a cytological level, to variation in the
osmolality of the ambient seawater, Ferraris (1979a1) proposed as one possible
mechanism a second order endocrine reaction (Fig. 4) in which the neurosecretory
system stimulates the release of acid mucosubstances from the cephalic gland of P.
spiralis and the cerebral organs of L. socialis
and A. lactifloreus into the vascular system
under hypoosmotic conditions. The apparent increase in release of secretory product
from the NSC and gland cells is followed
by the appearance of numerous sulfhydryl
containing droplets in the vascular system
or rhynchocoel (Ferraris, 19796, c, d). In
corroboration, Varndell (1981), in an
extensive histochemical study, found that
the acid mucopolysaccharide released into
the vascular system of L. ruber is catabolized into carbohydrate oligomers, and
possibly monomers, by carbohydrases
localized in the vascular endothelium. The
proposed mechanism is based on 1) the
absence of evidence for direct communication between the appropriate gland cells
and the nervous system (e.g., secretomotor
NEUROENDOCRINE DEVICES IN THE NEMERTINA
junctions), 2) the limitations inherent in
such conventional neural communication
(Scharrer, 1970), and 3) the fact that, classically, the neurosecretory neuron effects
such communication between the nervous
system and gland cells (Scharrer, 1967,
1970, 1972; Scharrer and Weitzman,
1970). First order endocrine reactions must
also be considered. In this case, the neuroendocrine devices would act independently and directly on target organs. An
investigation of the physiological significance of NSM and these acid mucosubstances, or at least the structures from
which they are derived, in the response of
nemertines to change in osmolality is discussed below.
Possible functions of nemertine neuroendocrine devices that are based on
physiological or behavioral evidence fall
primarily into three categories, i.e., reproduction, prey detection and volume regulation. With the exception of the work of
Bierne, much of this evidence is of recent
origin. In 1958, Gontcharoff and Lechenault demonstrated that the anterior
region of L. lacteus exerts an inhibitory
control over reproductive development.
Subsequently, through the use of selective
ablation and grafting techniques Bierne
(1966, 1970) identified the cerebral ganglia as the source of a gonad inhibiting
factor (GIF) in L. ruber. Continued investigation, using classical ablation-replacement therapy as well as autoradiographic
techniques, elevated GIF to the status of a
gonad inhibiting hormone (GIH), demonstrated that the cerebral ganglia in a
hoplonemertine A. lactifloreus similarly
regulates reproduction in both sexes, and
provided evidence that GIH acts through
inhibition of macromolecular synthesis in
sexual target cells (Bierne and Rue, 1979;
Rue and Bierne, 1980). Thus, removal of
GIH promotes RNA synthesis in young
oocytes, increases DNA synthesis in spermatogonia, induces DNA synthesis in primary spermatocytes and increases protein
synthesis in gametocytes and sex-specific
skin glands. These investigators suggest
that GIH may act as a regulating hormone
by inhibiting the secretions of sex-specific
stimulating substances produced by cells in
81
FIG. 4. Hypothetical regulatory mechanisms during
hypoosmotic exposure. Reproduced with permission
from Ferraris (1979d).
proximity to the gonads. Although GIH
has been localized to the cerebral ganglia
there is, as yet, no definitive evidence for
the cellular origin of the hormone.
Based on behavioral evidence, Amerongen and Chia (1982) propose a chemoreceptive function for the cerebral organs of
P. peregrina. They found that prey detection is abolished in this species following
surgical removal of the cerebral organs and
suggest that this form of cerebral organ
chemoreception may be common in the
Monostilifera. Such chemoreception would
not require that the cerebral organs function in a neuroendocrine manner, i.e., all
that need be involved is reception of the
appropriate stimuli via the nervous component of the organ. A chemoreceptive
function for the cerebral organs would also
be compatible with a role for these structures in response to variation in environmental osmolality. However, in the latter
case the cerebral organs would function
additionally as neuroendocrine devices.
Physiological evidence for a role of the
cerebral organs and cerebral ganglia dur-
82
JOAN D. FERRARIS
ing osmotic perturbation was first demonstrated by Lechenault (1965a, b). Intact
Lineus is capable of a regulatory volume
decrease, following initial swelling, under
hypoosmotic conditions. By employing
various ablation and grafting techniques,
Lechenault (1965a, b) determined that
when both the cerebral organs and the
cerebral ganglia are removed the regulatory volume decrease does not occur. He
also found that the presence of both structures is required for the appropriate time
course and extent of regulation. Similarly,
when Varndell (1981) ablated either the
cerebral ganglia or the cerebral organs in
the monostiliferan, A. lactifloreus, by electrocauterization, normal volume regulation was impaired. From Lechenault's
experiments it would additionally appear
that a nervous connection between the
cerebral organs and the cerebral ganglia
may not be required, since volume regulation occurs when these structures are
grafted in reverse order. A stimulus from
the cerebral ganglia to the cerebral organs
via vascular channels may, therefore, be
the mechanism involved if the reaction is
second order. Unfortunately, the parameters measured were not sufficiently
detailed to provide for separation of the
effects of the different structures involved.
were designed to determine the influence
of the cephalic glands and the cerebral ganglia on intracellular regulatory volume
decrease (in hypoosmotic media) as well as
regulatory volume increase (in hyperosmotic media). Based on changes in water
and inorganic solute content, neither the
cephalic glands nor the cerebral ganglia
play a role in intracellular volume regulation, regardless of the direction of the
osmolality change. Organic solutes, which
are almost exclusively involved in intracellular (vs. extracellular) volume regulation, are also unaffected by the absence of
these structures (Ferraris and Roderick,
1982). However, when the cerebral organs
and cerebral ganglia were removed from
Paranemertes peregrina (decerebrated), a
species in which the extracellular fluid volume is substantial (—40% of the total body
water content), volume regulation was significantly affected (Ferraris, 19846). Shamoperated animals, allowed to recover for
at least 5 days behave as do controls. Decerebrated P. peregrina initially gain significantly more water (g H 2 O/g solute free dry
weight = g H 2 O/g s.f.d.w.) than do controls during hypoosmotic exposure and
they retain significantly more water than
controls under hypoosmotic as well as fluctuating salinity conditions. The water content difference is accompanied by accuVolume regulation in the intact organ- mulation of Na and Cl (jimoles/g s.f.d.w.)
ism is some proportion of intra- and extra- in decerebrate worms. This occurs even
cellular phenomena. In L. ruber, where the under salinity conditions where loss of these
extracellular fluid volume is estimated as solutes would indicate volume regulation.
25% of the total body water content (Ling, The differences in water and ion content
1971), both neurosecretory and neuro- can be attributed to neuroendocrine conglandular structures have a significant trol of excretion as well as permeability to
effect on volume regulation. Since there electrolytes; however, one can not distinare no data, as yet, on the solutes involved guish between these effects from the
or the compartmentalization of the experiments performed. Normally, conresponse, there is no way of knowing which trol P. peregrina effect volume maintenance
aspects of volume regulation were affected via regulation of extracellular water and
in Lechenault's experiments.
solutes, but decerebrated P. peregrina are
More recently, experiments were car- not able to do this. Thus, while there is not
ried out on decerebrate and control P. spi- yet evidence for differential function
ralis, a species in which volume regulatory between the cerebral organs and the cereevents primarily reflect intracellular phe- bral ganglia (neurosecretory system?) it
nomena. Animals were subjected to diluted would appear that these devices stimulate
seawater as well as fluctuating salinity con- efflux of water and solutes at the nephridia,
ditions (Ferraris and Schmidt-Nielsen, or reduce ionic permeability of the integ1982; Ferraris, 1984a). These experiments
NEUROENDOCRINE DEVICES IN THE NEMERTINA
ument or gut, or both. Since so little is
known of nemertine physiology, one can
only select what appear to be the most logical ultimate target organs considering the
regulation involved.
Amerongen and Chia (1983) studied volume regulation, as percent of original wet
weight in P. peregrina, with and without
cerebral organs, under hypoosmotic conditions. In agreement with studies cited
above, animals deprived of the cerebral
organs gain more water than do control
worms. However, since their sham-operated animals eventually gained as much
water as did those lacking cerebral organs,
they attributed the observed effect to an
increase in integumental permeability
caused by the operation. The results by
Ferraris (19846) do not corroborate the
existence of a difference in permeability of
the integument to water among control,
decerebrate and sham-operated individuals since all groups demonstrate the same
rate of change in osmolality of tissue water
with exposure to a hypoosmotic medium.
Amerongen and Chia (1983) did not measure tissue water osmolality so no comparison is possible. Additionally, there is no
evidence for a difference in permeability
to ions between control and sham-operated
individuals (Ferraris, 19846). The discrepancy between the works cited may, however, reside in the time allowed for recovery after surgical procedures. The time
allowed by Amerongen and Chia (1983)
(24 hr or more) may be insufficient for
complete wound healing and reestablishment of the normal permeability of the
integument. This is unfortunate since comparison of the two works might have
allowed for some differentiation of function to be established for the cerebral
organs and the cerebral ganglia.
It is apparent that a great deal of work
needs to be done. The most important
questions at the present time involve 1) cellular localization and identification of the
gonad inhibiting hormone that is derived
from the cerebral ganglia, 2) separation of
the effects of the neurosecretory system
(cerebral ganglia) and neuroglandular
structures in volume regulation, 3) local-
83
ization of the target organs involved in the
volume regulatory response and 4) further
separation of the mechanisms involved in
extracellular volume control.
ACKNOWLEDGMENTS
Sincere thanks are given to Drs. Linda
H. Mantel and Janet Moore for critical
reading of the manuscript. This study was
supported by NIH awards GM 07047 to
Joan D. Ferraris and AM 15972 to Bodil
Schmidt-Nielsen.
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