PTEROPODS. 467 



species have beeii 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 181 1, during a tour 

 to the Orkneys, he observed on the rocks of the Isle of Staffa 

 several mutilated specimens of Clio borealis. Some days after, 

 having borrowed a large shrimp-net, and ro\\-ing 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 numerous ganglia are concentrated into a mass below the 

 oesophagus ; 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 small or obsolete; in Cleodora (Fig. 317) it is combined with the 

 fins ; but in Clio it is sufficiently distinct, and consists of two 

 elements ; in Spinalis 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 {epipodid) of the 

 sea-snails. The mouth of Pnetimodcrinon 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 



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