Z-oological Journal of the Linnean Society (1985), 84: 291-300. With 3 figures
Feeding mechanisms as evidence for
cyclostome monophyly
D. W. YALDEN
Department of <oology, The University, Manchester M13 9PL
Received March 1984, accepted for publication
Jub 1984
The feeding mechanisms of lampreys and hagfish, as described in the literature, are reviewed.
Although current opinion generally holds that these two groups have very different feeding
mechanisms, eleven synapomorphous features are recognized, and the underlying anatomical
homologies are suggested. I n these features, the two groups clearly share none of the anatomy of
gnathostomes, and it is concluded that the cyclostomes constitute a monophyletic group, the sistergroup of the gnathostomes.
KEY WORDS:-Lampreys
-
hagfish
-
monophyly - feeding mechanisms.
CONTENTS
Introduction . . . .
Lamprey feeding mechanisms
Tongue mechanism .
Skeleton . . . .
Musculature . . .
Hagfish feeding mechanisms
Tongue mechanism .
Skeleton . . . .
Musculature . . .
Discussion.
. . . .
Homologies . . .
Phylogenetic significance
. .
Acknowledgements
References.
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29 1
292
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INTRODUCTION
The cyclostomes are a small group of extant vertebrates of two basic types:
the lampreys, Order Petromyzontiformes, with 31 species in nine genera (Hubbs
& Potter, 1971) and the hagfish, Order Myxiniformes, with 21 species in five
genera (Adam & Strahan, 1963). Together with various fossil groups,
particularly the Pteraspida, Anaspida and Cephalaspida, they belong to the
Class Agnatha in traditional classifications. However, there is considerable
argument among specialists about the validity of the Class Agnatha as so
constituted. Were this to concern only the fossil vertebrates, incompletely known
0024-4082/85/070291+ 10 $03.00/0
29 1
0 1985 The Linnean
Society of London
292
D. W. YALDEN
and incompletely knowable, it would not be too surprising. What is surprising is
the lack of any real agreement over the relationships, close or otherwise,
between the lampreys and hagfish. The conventional view (e.g. Romer, 1966)
would place these two groups as agnathans, primitively jawless and distantly
related to jawed vertebrates, but at least closely related to each other; Schaeffer
& Thomson (1980), especially, have recently argued for this view. Most recent
authors, however, have argued instead that the lampreys are much more closely
related to the gnathostomes than to the hagfish, thus also implying that the
Cyclostomata (sensu Muller, 1836) and indeed the Class Agnatha are artificial
groups (Stensio, 1968; Lavtrup, 1977; Hardisty, 1979, 1982; Janvier & Blieck,
1979; Janvier, 1981; perhaps Halstead, 1982).
One part of the evidence for regarding the Cyclostomata as an artificial group
has been the assertion that the feeding mechanisms of hagfish and lampreys are
quite different; lampreys are characterized as having a ‘rasping tongue’, while
hagfish have ‘laterally biting jaws’. This characterization was apparently made
initially by Jarvik (1965, 1980), but has been generally followed. Janvier (1981:
125-127), for example, says that ‘despite vague resemblences, this device is now
regarded as independently acquired by each group’. It is my intention here to
demonstrate that the resemblances are quite detailed, and many; they surely
support the view that hagfish and lampreys are much more closely related to
each other than either group is to gnathostomes.
LAMPREY FEEDING MECHANISMS
I have carried out dissections and examined serial sections of Lampetra
Juviutilis to check what follows, but this account is largely based on, and uses the
terminology of, Hubbs & Potter (1971), Hardisty (1981) and Hardisty &
Rovainen ( 1982).
Tongue mechanism
Lampreys have a ‘tongue’ or ‘jaw’ with three rows of ‘teeth’ on it, a n anterior
transverse lingual tooth row and a pair of posterior lingual tooth-rows (Potter,
for example in Hubbs & Potter (1971), Potter, Hilliard & Bird, (1982), refers to
these as lingual laminae, but that does not seem to me an appropriate word -the tooth-rows are hardly flat plates). The posterior lingual tooth-rows diverge
posteriorly, and in a freshly dead animal they can be manipulated to close (or
open) towards (or away from) the midline of the tongue dorsally; in opening,
the tooth rows pivot anteriorly so that they diverge more abruptly (Fig. 1 ) . This
‘biting’ can also be observed in a live Lampetra.
Skeleton
The largest cartilage in the lamprey ‘jaw’ skeleton is the cartilugo pistoria, a
large, vertically orientated, flattened bar lying in the midline and ventrally
placed amongst the jaw musculature (Fig. 2 ) . Anteriorly this is continuous with
a zone of soft cartilage, and then with a pulley- or U-shaped cartilage, the
cartilago apicalis. The pulley or U-shape is manifest in ventral view; each arm has
FEEDING MECHANISMS AND CYCLOSTOME MONOPHYLY
Lampeha
A
293
Myxine
E
B
Antr.
c
/
F
7
/--
Figure 1. Diagram of the arrangement of ‘tooth-rows’ in lampreys and hagfish. I n anterior view, the
tongue apparatus of Lumpetru is visible in the mouth (A) as three lobes, bearing a median transverse
tooth row and a pair of longitudinal tooth rows (B). I n dorsal view, after dissection, the
longitudinal rows lie closely opposed in the closed position (C), but can be manipulated to open,
obliquely, from posteriorly ( D ) . In Myxine, the tooth-rows are similarly opposed in the closed
position (E), and open from posteriorly (F.)
a small bar of cartilage riding on it, the cartilago apicalis lateralis, to which the
posterior lingual tooth rows are attached. Another jaw cartilage, the cartilago
copulnris, lies ventral to the anterior end of the c. pistoria; it is a small roughly
triangular cartilage with the base anteriorly and the apex posteriorly directed.
Musculature
The principal jaw muscle, certainly the largest, is the m. cardioapicalis which
lies in the midline, dorsal to the cartilago pistoria. I t originates on the pericardial
cartilage, and inserts via a long tendon which forks anteriorly, each arm of the
fork inserting onto a cartilago apicalis lateralis. The m. cardioapicalis is invested
posteriorly in layers of circular muscle, the m. constrictor glossae. A pair of oblique
muscles, the mm. styloapicalis, originate on the styloid processes of the skull and
insert on the tendon of the m. cardioapicalis. The styloapicalis muscles act with
the m. cardioapicalis to retract the tongue and, because of the forked tendon, also
cause the posterior lingual teeth to bite together towards the midline (Fig. 2).
Protraction of the tongue apparatus is the result of activity by several ventral
muscles. These include a pair of mm. basilariglossus, running from the salivary
m iwsilariqlossus
Superficial
Lampetra
Deeper
m cardmpicalis
-
-
-
m. perpendicularis
m . calvatus
m. tubulotus
I st bronchial arch -
profundus
m.m. pmfmcmr:
dentium
m. pmmcmr
dentiurn
superficialis
C
Myxine
D
W
.e
N
FEEDING MECHANISMS AND CYCLOSTOME MONOPHYLY
295
glands back to the posterior tip of c. pistoria, and a pair of mm. copuloglossus rectus,
originating on the c. copularis and inserting on c. pistoria. The m. cornuoglossus and
m. annuloglossus are also tongue protractor muscles which insert on c. pistoria.
These muscles seem to act by pulling the c. pistoria forward between the
posterior lingual tooth-rows. The mm. copuloglossus obliquus, originating on the
c. copularis, run obliquely forward to insert on the posterior lingual tooth-rows,
and appear to me to act to open the tooth-rows (though Hardisty & Rovainen,
1982, consider them to be tongue retractors).
HAGFISH FEEDING MECHANISMS
This section is based largely on Cole (1905, 1907, 1912) and on Dawson
(1963); the terminology of the musculature in these two accounts is quite
different, and I have where possible followed Dawson as the more recent.
Tongue mechanism
Hagfish have two pairs of tooth-rows which diverge posteriorly, with much
sharper teeth than lampreys. They open from the rear, as well as away from the
midline, and when fully open face anteriorly and somewhat ventrally (Fig. 1).
As Dawson (1963) describes it, their action is rather like that of a book being
opened out flat and snapped closed; the ‘book’ faces antero-dorsally as it opens.
Skeleton
As described by Cole (1905), the skeleton of the jaw apparatus consists
especially of three large pieces of cartilage, the basal plates, lying mid-ventrally.
The largest of these, basal plate 3, is the posterior one, a vertical bar of cartilage
which is grooved dorsally and tapers posteriorly. Anteriorly, it is continuous
with a bar of mostly hard cartilage, basal plate 2. This in turn connects
anteriorly with basal plate 1, a gutter-shaped (in transverse section) or
somewhat pulley-shaped cartilage made up of three parts (Fig. 2). A U-shaped
or V-shaped (in transverse section) dental plate slides in the gutter of basal
plate 1, and supports the two pairs of tooth rows on its dorsal arms. It is a
complicated piece or cartilage, made up of two arches or bars, which folds about
its mid-ventral line when the tooth rows bite together.
Musculature
The principal jaw muscle, the m. clavatus, lies posterior to basal plate 3, and
somewhat dorsal to it, in the mid-ventral line. It inserts on the posterior arch of
the dental plate by a long tendon which runs fowards, in the groove of basal
plate 3, and fans out or forks just before its insertion. The m. clavatus is invested
in a sheath of circular muscle, the m. tubulatus, and is interrupted posteriorly by
a vertical sheet of muscle, the m. perpendicularis. The m. clavatus is the principal
____-__.
__-
Figure 2. Diagrammatic dissections of the tongue apparatus of lamprey (A, B) and hagfish (C,D) to
indicate some of the major structures. All drawn as from ventral dissections. More superficial views
(A,C) show protractor muscles and cartilages, deeper views (B,D) show retractor muscles, with
main cartilages and protractor muscles removed.
296
D. W. YALDEN
retractor of the tongue apparatus but also, because of its forked insertion, acts to
close the lingual tooth rows together in a transverse bite. The paired
mm. hyocopuloglossus, originating on the ‘first branchial arch’, insert directly
between the tooth-rows and dental plate.
The principal protractor muscles are the paired, double mm. protractores
dentium profundus which originate ventrally on basal plate 3, and pass anteriorly
to fuse into a tendon; this then runs anteriorly, round and over the pulley
formed by basal plate 1, to insert on the anterior margin of the dental plate. A
pair of rnm. protractores dentium supeq5cialis also originate ventrally on basal
plate 3, and sweep round the outside of the mm. protractores dentium profundus, as
they pass anteriorly to insert on the oral mucosa in front of the dental plate.
They help to pull the teeth foward and, because of their oblique pull, are
probably responsible for parting the tooth-rows; they certainly, as Cole ( 1907:
708) says, “tumble the whole apparatus” over the ventral lip of the mouth. I n
this, they aid the deeper muscles, which certainly, by their tendon, pull the
whole dental plate, and the tooth-rows with it, forward in the groove in the
dorsal side of basal plates 1 and 2. Another pair of muscles, the
mm. c o p u ~ ~ ~ u a d ~superficialis
atus
(as named in Cole, 1907) originate on the ‘dorsal
longitudinal bar’ or ‘pterygo-quadrate bar’ and run postero-ventrally to insert
on the dorsal margin of basal plate 3. The pair of mm. hyocopulopalatinus, which
originate partly from the cornual cartilages and partly from the fascia above
other muscles, also run postero-ventrally to insert partly on the basal plate,
mainly basal plate 2, though some fibres run back to the branchial arch. Both
copuloquadratus superficialis and hyocopulopalatinus muscles draw the basal
plate cartilages anteriorly and dorsally, moving the whole tongue apparatus
toward the mouth.
DISCUSSION
Homologies
The anatomy of the tongue apparatus in lampreys and hagfish is so unlike
any part of gnathostome anatomy that it is difficult for someone used to
gnathostome terminology to describe it and to understand it. Even terms like
tongue, jaws, teeth, and tooth-rows, which are useful descriptive terms in the
present circumstances, nevertheless have unfortunate implications of homology.
The most successful terminologies for the cyclostomes are relatively neutral ones,
then, which make no assumptions about homologies with gnathostome
anatomy. It is unfortunate, though, that quite separate terminologies have been
used for lampreys and hagfish; worse, in fact, because there are two quite
distinct sets of terminology used for hagfish anatomy (cf. Cole, 1907, with
Dawson, 1963) and also two sets in use for lampreys (cf. Lindstrom, 1949, with
Hardisty & Rovainen, 1982). This has greatly added to the difficulty of
understanding these systems, but it seems to me also to have obscured the basic
and essential similarity in the systems of lampreys and hagfish (Fig. 3). One can
enumerate at least 1 1 similarities: to emphasize the homologies, I give (in
parentheses) the appropriate name from lamprey anatomy followed by the
equivalent in the hagfish. Lindstrom (1949) reached very similar views on these
homologies.
FEEDING MECHANISMS AND CYCLOSTOME MONOPHYLY
297
Lamoefro
per.
m cop obl
Myxine
& I st
bronchial arch
m hyocopuloglossus
m.m profrocfor denfium
Figure 3. Diagrammatic lateral views of the tongue mechanisms of lamprey and hagfish, to
emphasize the basic similarity in plan. (Abbreviations: b.p. I-b.p. 3-basal plate 1-3; c. percartilago pericardialis; d.p.-dental
plate; m. cop. ob1.-m.
copuloglossus obliquus; m. cop. rectusm. copuloglossus rectus.
(1) The main skeletal support is a median ventral cartilage, a vertical bar
which tapers posteriorly (cartilago pistoria = basal plate 3 ) .
(2) This is continuous anteriorly with zone of soft (lampreys) or hard
(hagfish) cartilage (unnamed? = basal plate 2).
( 3 ) This in turn is connected with a more anterior pulley or U-shaped
cartilage (cartilago apicalis = basal plate 1 ) .
(4) There are one (lampreys) or two (hagfish) pairs of longitudinally aligned
tooth-rows; when parted or open, these face dorsally, and they close toward the
midline. (Lampreys also have a median transverse lingual tooth-row; this could
be homologous with the anterior pair of longitudinal tooth-rows in hagfish; in
which case, the posterior pair in hagfish would be directly homologous with the
pair of longitudinal tooth-rows in lampreys. Embryological evidence on the
origin of the transverse tooth-row in lampreys would be needed to resolve this
point.)
(5) The tooth-rows are carried on a pair (lampreys) or a single, complex
(hagfish) cartilage support (cartilago apicalis lateralis = dental plate).
(6) The main retractor muscle is a midline muscle which lies dorsal to and
extends posteriorly to the main skeletal support (m. cardioapicalis = m. clauatus).
(7) This muscle inserts by a long tendon which runs forward, dorsal to the
main skeletal support, forking to insert on the apicalis lateralis/dental plate.
(8) This main median muscle is invested in circular muscles ( m . constrictor
glossae = m. tubulatus).
(9) A pair of accessory retractor muscles, originating on cranial cartilages,
D. W. YALDEN
298
insert obliquely either on the tendon of the main retractor (lamprey) or onto the
dental apparatus generally (hagfish) ( m . styloapicalis = m. hyocopuloglossus) .
(10) T h e protractor muscles originate ventrally on the main skeletal support
(c, pistorialbasal plate 3). There are five pairs of these protractors in the
lampreys, and also five pairs in hagfish, but absolute homologies are uncertain.
The lamprey m. annuloglossus and m. basilariglossus, attaching to structures (the
annular cartilage and salivary glands) which have no counterpart in hagfish,
may well be two pairs which are missing in hagfish but I suggest that these have
a homologue in the hyocopulopalatinus. It seems that m. copuloglossus rectus
(lamprey) = mm. protractores dentium profundus (hagfish, both pairs), and the
m. cornuoglossus (lamprey) = m. copuloquadratus supe6cialis (hagfish).
(1 1) One pair of accessory protractors acts somewhat obliquely so as to part
the paired tooth plates ( m . copuloglossus obliquus = m. protractores dentium
supe6ciales).
This seems to me an impressive list of similarities, and the fact that nearly all
the muscles involved are innervated by the trigeminal nerve (Lindstrom, 1949)
may serve to emphasize that they are, as I suggest, homologous. The exceptions
are a few muscles innervated by the facial nerve (Lindstrom, 1949): ‘m. protractor
and posterior’
in
Myxine
(presumably
cartilaginis
basalis
anterior
m. hyocopulopalatinus and m. copuloquadratus supe$cialis
in Cole’s ( 1907)
terminology) and the ‘m. infravtlaire inferior’ and ‘m. vtlaribranchial’ in
Petromy<on (presumably m. cornuoglossus and perhaps m. cornuotaenialis in Hardisty
& Rovainen (1982, Table 11)
but in that case their statement of the
innervation is wrong for they ascribe a trigeminal innervation to all the piston
muscles).
~
Phylogenetic signijicance
The phylogenetic significance of these characters depends upon the likely
feeding mechanisms in ancestral vertebrates and in other (gnathostome)
vertebrates. I see no reason to doubt that the ancestral vertebrate was
microphagous, trapping food in a mucous sheet or web secreted by the
endostyle; this is the feeding mechanism of Cephalochordata, Ascidiacea, and
Thaliacea. I t follows that the retention of this feeding mechanism, albeit with
the ciliary pump replaced by a muscular one, in the ammocete larva of lampreys
is also the primitive condition, and indicates what feeding mechanism was
present in the earliest vertebrates. O n this basis, the feeding mechanism of adult
cyclostomes is a shared, derived (synapomorphous) feature of them. Equally
clearly, this mechanism, with its emphasis on antero-posterior protraction and
retraction, and on the transverse bite of the paired tooth plates, bears no
relationship to the essentially vertically biting jaws of gnathostomes. As
Schaeffer & Thomson (1980) remark, since the branchial skeleton of the
cyclostomes lies outside the gills, it is not homologous with that of gnathostomes,
and its nature is not compatible with the formation of functional jaws.
Thus in cladistic terminology, the cyclostomes are the sister group of
the gnathostomes, and neither group of cyclostomes has any close relationship
with
gnathostomes.
The
Cephalaspidomorpha
(conventionally
Petromyzontiformes Cephalaspida Anaspida) thus could include the
Myxiniformes, and be virtually synonymous with Monorhina, or the
+
+
FEEDING MECHANISMS AND CYCLOSTOME MONOPHYLY
299
Cephalaspidomorpha and Myxiniformes should be sister groups within the
Monorhina. As Schaeffer & Thomson (1980) indicate, we know nothing useful
about the relevant anatomy of the Pteraspida, but in particular we know
nothing whatever that would ally them with Myxiniformes, despite what both
Stensio (1968) and Jarvik (1980) argue. I n particular, the single pair of external
gill openings is not a shared character of these groups, since Bdellostomatidae
have 5-15 pairs of gill openings, and this multiple state is surely the primitive
condition for Myxiniformes. Thus Myxiniformes certainly do not belong in the
Pteraspidomorpha.
Since such conclusions are not consonant with those of recent reviews
(especially Hardisty, 1982), a few more general remarks on the relationship
between Myxiniformes and Petromyzontiformes are worth making. Hardisty
(1982) listed 81 characters common to these two groups and distinguishing them
from gnathostomes (including two references to the tongue mechanism). He also
listed 114 characters in which the two groups of cyclostomes differ; he felt that
the gnathostomes were approached more closely by hagfish in eight of these
characters, and by lampreys in 60. A large number of these relate, however, to
the more proficient osmotic and ionic regulation practiced by lampreys - not
only the seven listed under that heading, but many of those concerning higher
blood pressure and steroid hormones which are involved in kidney function.
Others are of doubtful significance in our present state of knowledge, and could
reflect our relative ignorance of hagfish, or are phylogenetically puzzling
(reduced eyes, eye muscles and nerves in hagfish, for example).
The Myxiniformes are all marine, are isosmotic with seawater, and appear to
be primitively so (Robertson, 1963); with the evolution of efficient
gnathostomatous predators in their marine environment, I suggest that they
replaced the vulnerable larval stage with direct development. This involved the
evolution of large, yolky eggs, surely an autapomorphous character (one evolved
also, quite independently, by Chondrichthyes, Coelacanthini, and Amniota) .
Conversely, the Petromyzontiformes have retained a microphagous, freeliving, ammocoete larva, their primitive character, but evolved protection for it
by moving into fresh water to breed. This required the evolution of improved
osmoregulation, their autapomorphous character. I t is true that, in doing so,
they acquired a number of characteristics which they share with some
gnathostomes, particularly some Actinopterygii, but I am not convinced that
they thereby indicate a closer relationship with gnathostomes than with hagfish.
0 ther gnathostomes (Chondrichthyes, Latimeria) have rather different ways of
solving osmoregulatory problems, and I consider that the resemblances between
lampreys and gnathostomes can well be due to convergent evolution.
In summary, I argue that the ancestral cyclostome was a marine vertebrate
with a microphagous larva and a predaceous adult. There are, of course,
significant differences between lampreys and hagfish in their tongue
mechanisms, as in all other areas of their anatomy. The hagfish have no
equivalent of the cartilago copularis, no transverse tooth-row but two pairs of
longitudinal tooth-rows, and a much more complex dental plate. It is not
possible to argue which, if either, more closely represents the ancestral
cyclostome condition. I would speculate that the presence of the cartilage copularis
is a derived lamprey feature, and that the transverse lingual tooth-row is derived
from a primitive paired condition. It is this transverse tooth-row that allows
300
D. W. YALDEN
lampreys to rasp their prey as well as bite it, and I suggest that this double
feeding action is also a derived state.
ACKNOWLEDGEMENTS
I should like to thank Mrs L. Kelly for preparing serial sections of Lampetra,
M r L. Lockey for photographic services and Mrs S. Hardman for typing. I am
also very grateful to those who have supplied reprints of their own work to
someone not previously concerned with their field of research.
REFERENCES
ADAM, M. & STRAHAN, R., 1963. Systematics and geographical distribution of Myxinoids. In A. Brodal &
R. Fange (Eds), The Biology of Myxine, pp. 1-8. Oslo: Universitetsforlaget.
COLE, F. J., 1905. A monograph on the general morphology of the myxinoid fishes, based on a study of
Myxine. Part I. The anatomy of the skeleton. Transactions of the Royal Society of Edinburgh, 41: 749-788.
COLE, F. J., 1907. A monograph on the general morphology of the myxinoid fishes, based on a study of
Myxine. Part 11. The anatomy of the muscles. Transactions o f t h e Royal SocieQ ofEdinburgh, 45: 683-757.
COLE, F. J , , 1912. A monograph on the general morphology of the myxinoid fishes, based on a study of
Myxine. Part V. The anatomy of the gut and its appendages. Transactions of the Royal Society of Edinburgh, 49:
293-344.
DAWSON, J. A,, 1963. The oral cavity, the ‘jaws’ and the horny teeth of Myxine glutinosa. In A. Brodal &
R. Fange, (Eds), The Biology of Myxine, pp. 23 1-255. Oslo: Universitetsforlaget.
HALSTEAD, L. B., 1982. Evolutionary trends and the phylogeny ofthe Agnatha. I n K. A. Joysey & A. E. Friday
(Eds), Problems of Phylogenetic Reconstruction, Systematics Association Special Volume 21: 159-196. London:
Academic Press.
HARDISTY, M. W., 1979. The Biology ofCyclostomes. London: Chapman and Hall.
HARDISTY, M. W., 1981. The skeleton. In M. W. Hardisty & L. C. Potter (Eds), Biology of Lampreys, vol. 3:
333--376. London and New York: Academic Press.
HARDISTY, M. W., 1982. Lampreys and hagfishes: analysis of cyclostome relationships. In M. W. Hardisty
& I. C. Potter (Eds), Biology of Lampreys, vol. 4B: 165-260. London and New York: Academic Press.
HARDISTY, M. W. & ROVAINEN, C. M., 1982. Morphological and functional aspects of the muscular
system. In M. W. Hardisty & I. C. Potter (Eds), Biology OfLampreys, vol. 4A: 137-231. London and New
York: Academic Press.
HUBBS, C. L. & POTTER, I . C., 1971. Distribution, phylogency and taxonomy. I n M. W. Hardisty & I. C.
Potter (Eds), Biology of Lampreys, vol. 1: 1-65. London: Academic Press.
JANVIER, P., 1981. The phylogeny of the Craniata with special reference to the significance of fossil
“agnathans”. Journal of Vertebrate Paleontology, I : 121-159.
JANVIER, P. & BLIECK, A., 1979. New data on the internal anatomy of the Heterostraci with general
remarks on the phylogeny of the Craniotes. Zoologica Scripta, 8: 287-296.
JARVIK, E., 1965. Die raspelzunge der Cyclostomen und die pentadactyle Extremitat der Tetrapoden als
Beweise fur monophyletische Herkunft. <oologische Anzeiger, 175: 8-143.
JARVIK, E., 1980. Basic Structure and Euolution o f t h e Vertebrates, vols 1 & 2. London and New York: Academic
Press.
LINDSTROM, T., 1949. O n the cranial nerves of Cyclostomes, with special reference to n. trigeminus. Acta
zoologica, 30: 315-458.
LBVTRUP, S., 1977. The Phylogeny of Vertebrata. New York: Wiley.
POTTER, I. C., HILLIARD, R . W. & BIRD, D. J., 1982. Stages in metamorphosis. I n M . W. Hardisty & I.
C. Potter (Eds), Biology ofLampreys, vol. 4B: 137-164. London: Academic Press.
ROBERTSON, J. C., 1963. Osmoregulation and ionic composition of cells and tissues. I n A. Brodal & R.
Fange (Eds), The Biology of Myxine, pp. 503-515. Oslo: Universitetsforlaget.
ROMER, A.S., 1966. Vertebrate Palaeontology, 3rd edition. Chicago: Chirago University Prrss.
SCHAEFFER, B. & THOMSON, K. S. 1980. Reflections on agnathan-gnathostome relationships. In L. L.
Jacobs (Ed.), Aspects of Vertebrate History: essays in honor of Edwin Harris Colbert, pp. 19-33. Flagstaff:
Museum of Northern Arizona Press.
STENSIO, E., 1968. The cyclostomes with special reference to the diphyletic origin of the Petromyzontida
and Myxinoidea. In T. Brvig (Ed.), Nobel gmposium 4: 13-71. Stockholm: Almqvist and Wiksell.