HYALEID^. 89 



which is either testaceous or membranaceous, whilst others are 

 naked. They all possess a heart, composed of auricle and ven- 

 tricle, within a pericardium. Their organs of sense are ver^^ 

 restricted : they have no eyes ; at least the little black points 

 formerly considered visual organs, M. Souleyet has ascertained 

 to be hearing pouches, having no exterior opening. The mouth 

 is more or less developed and is furnished with a lingual ribbon, 

 and the olfactory organ has its seat in the tentacles. 



The Cavolina tridentata oviposits at sunset. Its eggs are en- 

 veloped in a very smooth and elastic glairy ribbon, presenting a 

 series of pouch-like enlargements. The Thecosomoid species, 

 Cymbulia Peronii^ lays its eggs at any hour of the day: thej^ are 

 enveloped in a glairy cylindrical mass, containing a few parti- 

 tions or chambers, each of which ma}^ include forty eggs ; several 

 of these masses may be laid during a day, and the whole will 

 amount to about twelve hundred eggs. — FoL., Archives Zool. 

 Exp.^ iv, 1815. 



The larval pteropods are furnished with a velum, which dis- 

 appears a short time after the appearance of the adult swimming- 

 organs. In the earlier phases of their development a shell 

 alwaj^s exists ; even in those genera in which the adult is naked, 

 i. e. without shell. 



The Pteropoda are considered by some naturalists as a subor- 

 dinate group of the Grastropoda, and they are certainly much 

 more closely allied to the latter than to the Cephalopoda ; but 

 their pelagic habit and organization appear to indicate a distinct 

 class. Their geological record does not sustain the views of 

 those who look upon them as gastropods arrested in develop- 

 ment, for the type occurs in the primordial fauna ; moi^over, 

 they have a temporar}^ velum, so that the wings do not represent 

 that organ of the Gastropoda. 



Order THBCOSOMATA. 



Etym. — Theke, a case, soma, a bodj^ 



Animal furnished with an external shell, which is sometimes 

 cartilaginous ; head indistinct ; foot and tentacles rudimentary, 

 combined with the fins ; mouth situated in a cavity formed by 

 the union of the locomotive organs ; respiratory organ contained 

 within a mantle cavity, either dorsal or ventral. 



Family HYALEID^. 



Shell straight or curved, never spiral, globular or needle- 

 shaped, symmetrical. No operculum. 



Animal with two large fins, attached by a columellar muscle 

 passing from the apex of the shell to the base of the fins ; body 

 enclosed in a mantle ; gill represented by a transversely plaited 

 7 



