204 MR. MACDONALD
ON THE IIOMOLOOIEB OF TUX SO-CALLED
shallower, more oblique suture ; the pillar-lip is more dilated and
prominent at the base j and lastly, the peristome is not obtuse or
thickened and coloured. From all the other allied Madeiran
species it differs in its cylindric shape and short ovate aperture,
simply acute, and not narrowed or aeuminate at top. It is also a
considerably larger shell than A.gracilis and A. Leacociana, Lowe,
to which, amongst these other species, after A. prodwta, Lowe, it
most approaches j and from A. producta, Lowe, with which in shape
and size it best agrees, it is abundantly distinct by the form and
proportionate size of the aperture, besides the much less prominencc
of the pillar at its base. From A. foZ,?iczcks (Gron.) it differs in
the narrow turreted-cylindric shape, the short ovate aperture, and
the perfectly even and regular volutions (without any turgiduess
in the penultimate volution) of the spire.
--
On the Homologies of the so-called Univalve Shell and its Oper~
R.N., F.R.S., F.L.S.
culum. By JOHN D E N I MAODONALD,
Communicated by Prof. HUXLEY,
F.R.S.
[Read February 16th,1860.1
ADANSON
believed that the univalve spiral shell of the Gasteropod
was the homologue of the sinistral valve of the Conchifer, and
also that the dextral valve of the latter was represented by the
operculum of the former. Dr. Gray appears to support this view j
but as it has met with much opposition from recent writers, who
have substituted far more unnatural and fanciful theories, I
have been induced to draw up the present paper in support of the
doctrine, which I believe to be sound and philosophical, and suggestive of other remarkable homologies which must be ever concealed from our view while this primary barrier is permitted to
remain. I n dealing with this subject, it will be more convenient
to beg the question in the outset, andsustain it as well as itu. the
subsequent reasoning may be capable.
If Adanson’s position, aa above given, be right, it will be admitted that the left valve, 80 to speak, of the Gasteropod is more
generally that which receives the body of the animal j but in those
examples exhibiting a transposition of the viscera, and thcrefore a
sinistrdly spiral shell, the presumption is that the right valve
(normally the operculum) envelopes the animal, while the left
valve forms the operculum. As far as I have been able to discover,
thesc are the only cases in which a dextrally spiral opercululn
d00eS the aperture of II sinistral shell j for in these instances of
UNIVALVE SHELL AND ITS OPERCULUY.
208
Gasteropoda having a dextral shcll commencing with a sinistral
nucleus, the spiral nucleus of the operculum is also sinistral, and
in the spiral-shelled Pteropoda, the gyri of both shell and operculum are alike sinistral. The legitimate deduction in this latter
case is that the body-valve of the Pteropod, though sinistral, is the
equivalent of the dextral body-valve of the Qasteropod ; and both
we assume to be answerable to the left valve of the Conchifer. It
is sometimes stated that Atlanta affords an exceptional example of
a dextral shell with a dextral operculum" ; but this I can confidently affirm is not the case ; the operculum of Atlanta is sinistral,
like all normal opercula.
As relates to the umbones of bivalves and their homolopes in
the shells of Gasteropoda, the spiral nucleus of both body-valve
and operculum is very significant. From tho examination of
numerous species of the operculated genera of Heteropoda captured
from time to time in the Atlmtic and the Pacific, I discovered that
in all those shells possessing a prominent spiral nucleus, the corresponding little point of the operculuni was always beautifully
curved and proportionately well expressed ; but when the nucleus
of the shell wa8 involute, the nucleus of the operculum presented
a most indefinite and irregular appeara.nce,though still obviously
disposed in a spiral fashion. This latter caae is illustrated in the
genus O~zygyru8(Benson), characterized by having a more or less
horny shell with an involute nucleus.
The study of the development of the Conchifera, and the nice
examination of the relations and connexions of the mantle, tend
to show that that part of the latter which line8 the right valve is
quite distinct from that which lines the left; and the primitive
distinctness of these two portions is permanently exemplified by
their homologues in the operculated Gasteropoda.
Any one who will examine the operculigerous lobe in the genus
Turbo, cannot fail to recognize the idea of an opercular mmtle, the
source of that remarkable deposit of shelly matter which so distinguishes Turbo, Phasianella, and other genera of the same family.
The free margins of this opercular mantle are often ample enough
to meet over and conceal the operculum completely, and they may
be traced in continuity like those of the body-valve mantle round
what I have been induced to call the ('pedicle," which encloses the
retractor or adductor muscles, and connects each mantle respectively with tlic body of the foot and the investment of the visceral
mass.
IL
Woodnn~~l's
Mniiiinl of Molluscn, p. 201.
20G
MR. YACDONALD ON TIIE IIOMOLOOIES OF THE SO-CALLED
There is no example of a Conchifer in which the right-mantle
margin does not blend with the left, at least in the region of the
hinge ; while, on the other hand, there is no inAtance amongst Gasteropods in which the margin of the right or '(opercular mantle "
coalesces with that of the left or " shell mantle " at any point.
Here, then, is a simple feature distinguishing two Orders of Mollusca, arising out of the determination of the homology of the univalve and operculum of the Gasteropod with the bivalve of the
Conchifer.
Preserving their homology with the two adductors of '' dimyary
Coi~h@ra," and perhaps also combining the representatives of
those fibres which serve to withdraw the foot, the great retractor
muscles of Basteropoda generally consist of two principal fascicull;
and even in the shell-protected embryo of those species which are
destitute of a shell in the mature state (e. g. Eoh) the temporary
retractor is composed of two distinct bundles. The double nature
of this muscle is usually more apparent at the opercular extremity
than at the opposite end, as in the beautiful genus Atlanta for
example, in which the retractor muscle, arising by a broad oblique
attachment extending some little distance upon the right or upper
wall of the tube, is inserted into the operculum by two distinct
fleshy slips (the great " pedal sinus " intervening), while certain
portions diverge into the vertical f
h and sucker-disc. A still
better example occurs in the genus Neritinu, in which a wide space
intervenes between the two retractors. The inner extremity of
these muscles scarcely rises above the level of the general surface
of the mantle, presenting a broad smooth facet on either side of the
body. Indeed the corresponding muscular impressions in the
bowl of Navicella shadow forth most unequivocally those of the
anterior and posterior adductors in Dimyary bivalves. The correctness of this view is supported by the fact that the fibres composing the lateral muscles may be traced through the operculigerous lobe towards the operculum, into which they are separately
inserted. The articulation of the operculum of Neritina with the
columella of the shell is retrospective, as it were, of the utate of the
typical bivalve, while in other respects the higher type of the
Gasteropod is more strikingly developed,
The theoretical conversion of the left valve of a Conchifer into a
spiral dextral shell, with a rudimentary and modified right valve
forming a suitable operculum, requires no great stretch of the
imagination to conceive. The retractor muscles of Gasteropoda
passing between the shell and the operculum, take up a longi-
UNIVALVE SIIELL AND ITS OPEBCULUM.
207
tudinal direction with respect to the animal ; while the shells of
Conchifera, being applied to the sides of the body, necessitate the
transverse direction of the adductors. But we shall have little
occasion to disturb the latter from their position while we attempt
to follow those transitional steps by which, in theory, we may trace
the conversion of the physical condition of the Concliifer (in harmony with the slight metamorphosis of its testaceous covering)
into that of the Qasteropod. If, then, while the shell-metamorphosis is going forward we suppose the body of the Conchifer, now
occupying its left valve, to revolve, irrespective of the adductors,
from left to right on its longitudinal and transverse axes, in both
capes moving through a quarter of a circle, the foot and oral
extremity at the same time undergoing certain changes (the latter
especially, by the acquisition of new parts), the characters of the
resulting being will closely resemble those of Nauicella, to which
we have already alluded.
All the external parts of the Qasteropod or the Conchifer
appear to be superimposed, as it were, upon each other in a definite
order, and so disposed as to effect a more or less perfect bilateral
symmetry. Proceeding from above downwards, they admit of
arrangement into four distinct systems, as in the following Table.
The principal openings are the Oral, Anal, and Generative,-the
Oral and Generative between the '' ba&podial" and " epi-podial "
systems, and the Anal between the gills in the Gasteropod, in which
therefore all are in advance of the retractors. In the Conchifer,
on the other hand, the epipodial system being absent, the oral
orifice lies between the gills, or rather their anterior appendices,
below the anterior adductor, the anal orifke opens between the two
adductors, and the generative openings are below the posterior
adductor. The particulars just mentioned constitute the more
essential points of difference between the Gaeteropod and the
Conchifer ;but the subject will be better understood by reference
to the Table. [See next page.]
Although an abdomen may be said to exist in all cases, it can
scarcely be regarded as a distinct part of the animal, being merely
the space included by all the external organs and containing the
viscera ;and the " soma " or body is formed by the union of all these
organs enclosing the abdomen. I n the case of the Uaateropod
therefore, the viscera are contained in the left-mantle pedicle, protected by the left valve, which becomes concave to receive them.
The right-mantle pedicle, bearing the operculum. from its nsual
position has hitherto been mistaken as a part of the foot, and called
208
IIOMOLOOIES OF UNIVALVE BEELL AND OPERCULUM.
metapodium ;” and as the “ mesopodiwm,” so cnlled, derives its
name from its intermediate position between the left-mantle
pedicle and the “propodizum,” it is so far incorrectly named; I
therefore substitute for it the term “ basirpodizcm.”
*
I‘
Common tabwlar Plarlt of the Organization of Gasteropoda and
Conchifera, showing also their points of @Terence.
___.__
j
Systems of external organs,
enclosing the abdomen, b.
1
hctions.
I
Relationship of
internal systems.
I. PALLIAL.
Access0 to respira1. shells and ligaments.
2. Pallial lamina and pediclea. tion mTdefensive.
1
~
~~
’
Anal aperture in Conchifera betwcen the adductom and generativc
openings, below the posterior adductor.
~~
11. BRANCIIIAL.
1. Branch& proper.
,
2. Anterior appendages or
Cirri.
-.
i
I
I
-
Oral aperture of Conchifera and a n d sperture of (fasteropoda
between the gills.
~~
~
I
1111. EPIPODIAL.
’
~
Circulatory and
depurative.
Itaspiration.
,
1. Epipodia proper.
2. Superior tentacda.
IV. BASIPODIAL.
1. Badpodium.
2. Propodium.
1
Nervous systen
Natation and mnaaand
tion.
Organs of sense
Prehension and locomotion.
Generative.
With the above principles before us, the doctrine that the univalve shell, irrespectivo of the operculum, is the equivalent of
both valves of the Conchifer united together, is quite untenable ;
nor does the prima facie comparison of Lepton spamoszum with
Trochus (such having been chosen by Mr. Woodward in support
of this view) establish it0 truth one whit. It must be remembered
also that it devolves upon the upholders of this improbable piece of
transcendentalism to dispose of the operculum, which in many cases
assumes almost as much importance as the shell itself. Many
conchologists regard the operculum merely as a glutinous or
horny secretion, sometimes the nidus of calcareous deposit, adapted
to the form of the mouth of the shell as a further protection to
the retracted animal ; but beyond this, in their estimation, it
QYRODAUTYLUSELEQANS OCCURRING ON BTICKLEBACK~.
209
deserves little consideration, except from the necessity of noting all
the varieties of its form and structure, by which they are enabled
to fit the appropriate doors to the many empty dwellings in their
cabinets.
The supposition that the particular part of the foot occupied by
the byssus in some bivalves corresponds with that which supports
the operculum in Gasteropods, is just as unphilosophical as the
notion that the vesicular float of I i t h i n a is an extreme modification of the operculum. Nor is the mass of byssus which closes up
the interval between the valves of the unattached Bysso-area to bo
regarded as the homologue of the operculum in Qasteropods,
although it m y fulfil an analogous function. There are juster
grounds for believing that the float of I i t h i n a , Xacgillivrayia, &c.,
the suspensory threads of Litiopa, Planaxis, &c., the temporary
byssus of the young Anodon, Naia, and Uyclae, and the permanent
byssus of other bivalves, fixed or unattached, are all essentially
equivalent structures, having a local origin altogether distinct from
the " operculigerous lobe " 8 s it exists in Gasteropods, or from what
I am induced to regard as its homologue in Conchifers.
On the Occurrence of CJyrodactylUs elegans on Sticklebacks in the
Hampstcad Ponds, January 1860. By C.L.BBADLEY,
Esq., F.L.S.
[%ad February 16th, 1860.l
IN examining some of the common Sticklebacks obtained from the
Hampstead Ponds during the present month, I found them infested
with numerous minute parasitic worms. Although these were
more conspicuous upon the fins, they were scattered over the
general surface of the skin, and were attached by one end to the
fish, while the other floated freely. The parasite has the external characters of a suctorial Annelid. The body is subcylindrical, annulate, without setse or cilia, and terminated posteriorly
by a suctorial base. In its mode of progression, in the expansion
and contraction of the body, it closely resembles the Leech. When
fully stretched out, it measures about &th of an inch in length, and
about &th in breadth, tapering from the middle towards both ends,
the cephalic portion being by far the narrower. The latter is bifid,
each division furnished with a retractile brush-like extremity,
which the animal uses as a tactile organ and also for progression.
The posterior base is of a horse-shoe form ; the curved margin is
divided into sixteen digitntioiis, each having an independent niove-