HYALEIDZ. 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 very 
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 Peronit, lays its eggs at any hour of the day: they are 
enveloped in a glairy cylindrical mass, containing a few parti- 
tions or chambers, each of which may include forty eggs; several 
of these masses may be laid during a day, and the whole will 
amount to about twelve hundred eggs.—Fou., Archives Zool. 
Bip Al S15: 
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 
always exists ; even in those genera in which the adult is naked, 
7. e. without shell. 
The Pteropoda are considered by some naturalists as a subor- 
dinate group of the Gastropoda, 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; moreover, 
they have a temporary velum, so that the wings do not represent 
that organ of the Gastropoda. 
OrperR THECOSOMATA. 
Etym.— Theke, a case, soma, a body. 
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. 
Famity 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 
Us 
