442 THE OCEAN WOKLD. 



flappers, and descend to such a depth in their watery world as will 

 give them the security they seek. They thus pass their lives in the 

 open sea far from any other shelter, except that yielded hy the gulf 

 weed and other algae. In appearance and habits, these small and 

 sometimes microscopic creatures resemble the fry of some other forms 

 of mollusca. They literally swarm both in Tropical and Arctic seas ; 

 sometimes so numerous as to colour the ocean for leagues. They are 

 the principal food of whales and sea-birds in high latitudes, rarely 

 approaching the coast. Only one or two species have been acci- 

 dentally taken on our shores, and those evidently driven thither by 

 currents into which they have been entangled, 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 Clio lorealis. 

 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 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 lanceolata (Fig. 309) it is combined 

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

 elements or spirals ; the superior 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 (Epipoda) of the sea- 

 snails. The mouth of Pneumodermon is furnished with two sup- 

 porting 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 resemblance 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 



