3 66 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- 

 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, Spongiobranchcea, Pneumoderma, Clionopsis, Noto- 

 branchcea, Clione, and Halopsyche, Dr. Pelseneer locates them next to the Aplysiidce. 



