366 
MOLLUSCS. 
carnivorous in their habits, and live upon minute animals which abound in the sea, 
including small Mollusca, and Entomostraca and other Crustacea. Some of them 
possess prehensile organs for seizing their prey, but many appear to be totally 
unprovided with any special appendages for this purpose. In the early stages 
of development a shell always exists, but when adult, only certain forms are 
furnished with such protective covering, the others being naked molluscs. They 
possess a heart, consisting of auricle and ventricle, enclosed within a pericardium. 
The branchiae are either internal or external. The head, distinct in some groups, 
and furnished with one or two pairs of tentacles, is practically wanting in others, 
and reduced to a mere mouth at the base of the fins. Nearly all the various forms 
have a radula, and some horny jaws. There are no eyes, or these are only repre¬ 
sented by minute pigment dots upon the visceral sac, or on the tentacles. The 
fry of the Pteropods closely resembles that of ordinary Gastropods; but sub¬ 
sequently the frontal veil of the Pteropod disappears, and is replaced by the 
parapodia or permanent fins. 
The number of species of this group is inconsiderable, and may be estimated 
at about fifty-four; but, as regards individuals, the numbers are inconceivable. 
The bottom of the sea in various parts of the Gulf of Mexico, the Bay of Biscay, 
and the Mediterranean, is paved with an accumulation of the dead shells of 
Cavolinia, Cleodora, and other forms. Their tiny forms occur in shoals in the 
tropics, and in more temperate seas, and, even in Arctic latitudes, they exist in 
such myriads as to discolour the water for considerable distances. They there 
form a large item in the diet of the Greenland whale. Some of the species have a 
wide distribution, occurring in the Atlantic, Pacific, and Indian Oceans, others are 
more localised. 
The Pteropoda are divided into two sections, namely, Gymnoso- 
Gymnosomata. x " 
mata and Thecosomata. The animals belonging to the former group 
have no mantle or shell in the adult state. The body is generally elongate, the head 
distinct, and furnished with two pairs of tentacles, the posterior bearing rudimentary 
eyes. The swimming-lobes are attached to the sides of the neck, which is somewhat 
narrowed and constricted. The breathing-organ is external, lateral, or at the 
posterior end of the body. The foot is rudimentary and ventral. The radula is 
composed of numerous rows of teeth as in some of the Nudibranchs. The young 
at first are contained in a minute straight shell, and swim by means of a ciliated 
frontal veil (velum). Subsequently these structures disappear, and the larva pro¬ 
gresses by means of circles of cilia which surround the body at intervals. In the 
final metamorphosis, the cilia disappear, the parapodia are developed, and the animal 
assumes the adult condition. The gymnosomatous Pteropoda are only few in number 
of species, but, like the rest of the group, occur sometimes in enormous numbers, and 
are very widely distributed. All are small creatures, the largest not exceeding an 
inch and a half in length; they are carnivorous, and often, it is said, feed upon 
their shell-bearing relatives. The position assumed by the Gymnosomata when 
swimming is vertical, with the head uppermost, or else slightly sloping. 
The Gymnosomata are arranged in five families, comprising only seven genera, 
namely, Dexiobranchcea, Spong iobranchcm, Pneumoderma, Clionopsis, Noto- 
branchcea, Clione, and Halopsyche. Dr. Pelseneer locates them next to the Aplysiidai. 
