PTEROPODA. 347 
In structure, the Pteropoda are most nearly related to the 
marine univalyes, but much inferior to them. Their nervous 
ganglia are concentrated into a mass below the cesophagus; they 
haye auditory vesicles, containing otolites; and are sensible of 
light and heat, and probably of odours, although at most they 
possess very imperfect eyes and tentacles. The true foot is 
small or obsolete ; in cleodora it is combined with the fins, but 
in Clio it is sufficiently distinct, and consists of two elements ; 
in Spirialis the posterior portion of the foot supports an oper- 
culum. The fins are developed from the sides of the mouth or 
neck, and are the equivalents of the side-lappets (epipodia) of 
the sea-snails. The mouth of Pnrewmodermon is furnished with 
two tentacles supporting miniature suckers; these organs have 
been compared with the dorsal arms of the cuttle-fishes, but it 
is doubtful whether their nature is the same.* A more certain 
point of resemblance is the ventral flexure of the alimentary 
canal, which terminates on the under surface, near the right 
side of the neck. The pteropods havea muscular gizzard, armed 
with gastric teeth; a liver; a pyloric cecum; and a contractile 
renal organ opening into the cavity of the mantle. The heart 
consists of an auricle and a ventricle, and is essentially opistho- 
 branchiate, although sometimes affected by the general flexure 
of the body. The venous system is extremely incomplete. The 
respiratory organ, which is little more than a ciliated surface, is 
either situated at the extremity of the body and unprotected by 
a mantle, or included in a branchial chamber with an opening 
in front. The shell, when present, is symmetrical, glassy, and 
translucent, consisting of a dorsal and a ventral plate united, 
with an anterior opening for the head, lateral slits for long fili- 
form processes of the mantle, and terminated behind in one or 
three points; in other cases it is conical, or spirally coiled or 
closed by a spiral operculum. The sexes are united, and the 
orifices situated on the right side of the neck. According to 
Vogt, the embryo Pteropod has deciduous vela, like the sea- 
snails, before the proper locomotive organs are developed. 
(Huxley.) 
From this it would appear that while the Pteropoda present 
some analogical resemblances to the Cephalopoda, and perma- 
nently represent the larval stage of the sea-snails, they are 
developed on a type sufficiently peculiar to entitle them to rank 
* The figures of Eydoux and Souleyet represent them as being supplied with nerves 
from the cephalic ganglia ; whereas the arms of the cuttle-fish, and all other parts ot 
modifications of the foot in the mol/usea, derive their nerves from the pedal ganglw. 
(Hunley, ) 
