PTEROPODS, 467 
species have been accidentally taken on our shores, and those 
evidently driven thither by currents into which they have been en- 
tangled, or by tempests which have stirred the waters with a 
power beyond theirs. Dr. Leach states that in 1811, during a tour 
to the Orkneys, he observed on the rocks of the Isle of Staffa 
several mutilated specimens of Cio borealis. Some days after, 
having borrowed a large shrimp-net, and rowing along the coast of 
Mull, when the sea, which had previously been extremely stormy, 
had become calm, he succeeded in catching one alive, which is now 
in the British Museum. 
“In structure,” Mr. Huxley tells us, “the Pteropods are most 
nearly related to the marine univalves, but much inferior to them. 
Their numerotis ganglia are concentrated into a mass below the 
cesophagus ; they have auditory vesicles containing otolithes, 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 smali or obsolete ; in Cleodora (Fig. 317) it is combined with the 
fins; but~in Co it is sufficiently distinct, and consists of two 
elements ; in Spzria/is the posterior portion of the foot supports an 
operculum. The fins are developed from the sides of the mouth or 
neck, and are the equivalents of the side-lappets (e¢zpodia) of the 
sea-snails. The mouth of Pxeumodermon is furnished with two 
tentacles supporting miniature suckers; these organs have been 
compared to the dorsal arms of the cuttle-fishes ; but it is doubtful 
whether their nature is the same. A more certain point of resem- 
blance is the ventral flexure of the alimentary canal, which terminates 
on the under surface near the right side of the neck. The Pteropods 
have a muscular gizzard armed with gastric teeth, a liver, a pyloric 
caecum, and a contractile renal organ opening into the cavity of the 
mantle. The heart consists of an auricle and a ventricle, and is 
essentially opisthobranchiatic, although sometimes affected by the 
general flexure of the body. The venous system is extremely in- 
complete. The respiratory organ, which is little more than a ciliated 
surface, is either situated at the extremity of the body, and unpro- 
tected 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 filiform 
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 are situated on 
the right side of the neck. According to Vogt, the embryo Pteropod 
EE 2 
