3336 



THE MOLLUSKS OR SHELLFISH 



Thecosomata 



tinct, 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 some- 

 what 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, Dexiobtanchcea, Spongiobranchcea, Pneumoderma, Clionopsis, Notobranchcea, 

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

 The forms belonging to this section of Pteropoda are characterized by 

 the presence of a delicate external shell, by the foot being repre- 

 sented only by the two anterior symmetrical fins, by the presence of a mantle, 



by the absence of eyes, and (except in a 

 few cases) of a gill. The head is indistinct 

 and furnished with a single pair of 

 tentacles. The mouth is unprovided with 

 hook sacs, as in the Gymnosomata. The 

 radula has only a single lateral tooth on 

 each side, with a small basal piece. The 

 Thecosomata are divided into the three 

 families, Limacinidce, Cymbuliidce, and 

 Cavoliniidce. 



The members of the family Lima- 

 cinidce possess a small sinistrally coiled 

 shell, provided with a delicate glassy 

 operculum, which is attached to the 

 posterior lobe of the foot. The animal 

 can withdraw completely within the shell, 

 which, when the animal is swimming, has 

 a lateral position, or possibly rests with 

 the spire inclining somewhat downward. 

 Two genera are included in this family, 

 namely, Limacina and Peradis. In Lima- 

 cina, of which ten species are known, the 

 I.ARVA OF Pneumoderma (magnified). shell is rather globose, umbilicated, with a 



