MOLLUSCA. 
354 
Mollusca. © Genus Exepona.—Suckers on the arms disposed in a 
—-\-~" single row. 
M. Lamark has figured and described two species of this 
genus, in the Mém. de la Soc. d Hist. Nat. One of these is 
a native of the Mediterranean, and is remarkable for giving 
out an odour like musk. 
b. Arms unequal. 
Genus OcytHor.—Two of the arms at their inner ex- 
tremities furnished with membranaceous expansions. 
In this genus, which was instituted by M. Rafinesque, 
the suckers are in a double row, and supported on foot- 
stalks. In the specimens of the Ocythoe Cranchii, pro- 
cured during the expedition to the Congo, Dr. Leach ob- 
served “four oblong spots on the inside of the tube, re- 
sembling the surfaces for the secretion of mucus, two in- 
ferior and lateral, and two superior, larger, and meeting an- 
teriorly. On the rim of the sac, immediately above the 
branchiz, on each side, is a small, short, fleshy tubercle, 
which fits into an excavation on the opposite side of the 
sac.” (Phil. Trans. 1817.) 
This animal was long considered as the fabricator of the 
shell termed Argonauta or Paper Nautilus. The observa- 
tions of Mr. Cranch, the zoologist to the Congo expedition, 
were supposed to have demonstrated that the shell is merely 
the temporary residence of this animal, which it quits at 
pleasure. The body of the animal does not conform in 
shape to the cavity of the shell, nor to all its irregularities 
of surface ; neither is there any muscular attachment be- 
tweenthem. “On the thirteenth of June,” (says Dr. Leach, 
when publishing the notes of Mr. Cranch,) “he placed two 
living specimens in a vessel of sea-water ; the animals very 
soon protruded their arms, and swam on and below the sur- 
face, having all the actions of the common Polypus (oc- 
topus) of our seas ; by means of their suckers, they adhered 
firmly to any substance with which they came in contact, 
and when sticking to the sides of the basin, the shell might 
easily be withdrawn from the animal. They had the power 
of completely withdrawing within the shell, and of leaving 
it entirely. One individual quitted its shell and lived seve- 
ral hours swimming about, and showed no inclination to re- 
turn into it; and others left the shells as he was taking them 
up in the net. They changed colour, like other animals of 
the class Cephalopodo; when at rest, the colour was pale 
flesh-coloured, more or less speckled with purplish; the 
under parts of the arms were bluish-grey ; the suckers whit- 
ish.” The specimens which furnished an opportunity for 
making the preceding observations, were met with in the 
Gulph of Guinea, and afterwards on the voyage, swimming 
in a small argonauta, on the surface of the sea. The reader, 
who is desirous of farther information on this subject, may 
consult Dr. Leach’s Observations on the Genus Ocythoe of 
Rafinesque, Sir E. Home on the Distinguishing Characters 
between the Ova of the Sepia, and those of the Vermes Tes- 
tacea that live in water, in the Philosophical Transactions 
for 1817, art. xxii. and xxiii., (both of which are added to 
the appendix of Captain Tuckey’s Narrative,) and a paper 
by Mr. Say, on the genus Ocythoe, in the Phil. Trans. 
1819, art. vii. More recently, however, naturalists seem 
disposed to reunite the Ocythoe with the Argonauta, though 
the question cannot be considered as determined. 
Cuass I1—Preropopa. 
Fins formed of membranaceous expansions. 
This class was instituted by Cuvier, for the reception of 
a few genera, the peculiar characters of which indicated the 
impropriety of suffering them to remain in any of those 
categories which had been previously established. All 
the species are small in size; and the attempts hitherto 
made to investigate their internal structure, have, in a great 
measure, failed in explaining the functions of the organs Mollusca. 
which are exhibited. The valuable papers of Cuvier, on === 
the Clio, Pneumodermon and Hyalea, include nearly all the 
accurate information on the subject, of which naturalists 
are in possession. 
The general form of these animals is somewhat ovate. 
The tunic appears in some genera, as the Clio and Pneumo- 
dermon, to be double, the external one soft and thin, the 
internal exhibiting a fibrous structure, corresponding to the 
muscular web of the skin of the higher classes. In these 
animals, however, the two layers are unconnected through- 
out the greater part of their expansion. In some, as the 
Cymbullia, the tunic is cartilaginous, while in others it is 
strengthened by a shell. In these last, the shell in the Li- 
macina is a spiral univalve, covering the abdominal viscera, 
and in the Hyalea, where it serves the same purpose, it ap- 
proaches in character a bivalve shell. It is, however, des- 
titute of a hinge, the two valves being united together at 
their caudal margins, and there is no appearance of a trans- 
verse adductor muscle. 
The organs of motion in all the genera consist of two fins, ” 
or membranaceous expansions, one being seated on each 
side of the head. They have no foot wherewith to crawl, 
nor any suckers by which they can adhere to objects. They 
are, therefore, free animals moving about in the water by 
means of their fins, and probably possessing, at the same 
time, a power of varying their specific gravity, as they are 
capable of varying, to a certain extent, the form of their 
bodies, and of enlarging or reducing their dimensions. There 
is nothing peculiar in their nervous system. 
The organs of digestion differ greatly from those of the 
Cephalopoda, which we have already considered. They are 
generally regarded as destitute of eyes and ears. Their 
tentacula are either seated on the head, forming two com- 
plicated branches of filaments, or spread along the margin 
of the tunic. There are no arms for seizing the food. The 
mouth, however, is furnished with lips; and in some there is 
an appearance of a tongue at the entrance of the gullet. The 
salivary glands are two in number, lengthened, descending 
a considerable way into the abdomen, and pouring their 
contents, by means of their excretory canals, into the cavity 
ofthe mouth. The gullet, after being encircled by the ner- 
vous collar, suffers an enlargement, which has been termed 
a crop, contiguous to which is the stomach. Both these 
cavities exhibit muscular ridges on the inner surface. The 
liver surrounds the stomach, is intimately united with its 
contents, and pours in its bile by numerous pores. The in- 
testine is short, and, after making one or two turns, ascends 
and terminates in the neck near the mouth. 
The circulating system in this class has been but very 
imperfectly investigated. The pulmonic vessels are un- 
known, but systemic veins, a single auricle, ventricle, and 
aorta, have been detected.- The heart, in some, is situate 
on the left, in others, on the right, side of the body. 
The aérating organs exhibit very remarkable differences. 
Tn the Clio they are in the form of a fine net-work on the 
surface of the fins; in the Pneumodermon they are conjec- 
tured to form leaf-like ridges on the caudal extremity of the 
body ; or if these ridges are to be considered as particular 
kinds of fins, the gills may be sought for on the membrana- 
ceous expansions of the neck. In the Hyalea the branchize 
form a complex band on each side of the, body, at the lateral 
opening of the shell. 
The animals of this class are all hermaphrodites. There 
is a common cavity, a vesicle, penis, vas deferens, and tes- 
ticle, together with an oviduct and ovarium. These open 
near the mouth on its ventral margin. There is nothing 
known with respect to the appearance of the eggs, the pe- 
riod of propagating, or the form of their young. 
All the animals of this class inhabit the sea. Some, as the 
