Mollusca. 



81 



then the other, the body helping with violent writhings. They live only 

 a few weeks in captivity. 



Equally remarkable but of smaller size is Aeolis (Fig. 102). Doris 

 also (Fig. 100) with its dorsal tufts of feathery gills belongs to this 

 group. Frequently they may be observed laying eggs, which are generally 

 enclosed in a mucilaginous mass and arranged in ribands or strings. 



Allies of the snails form a portion of the pelagic fauna (see Note 

 on p. 62). Swimming among the transparent Medusae and Ctenophora, 

 we have the curious Keeled Snails (Heteropoda) and Sea-butterflies (Ptero- 

 poda). Especially in spring and in autumn, when the sea is full of pe- 

 lagic life, these two groups make their appearance; in the Aquarium, 

 however, they only last a very short time (Tank 20) and belong therefore 

 to the rarer guests. Of the Heteropoda we would mention Pterotrachea 

 (Fig. 170), a perfectly transparent, long and thin animal with a curved 



Fig. 170. Pterotrachea coronata. l /- 2 nat. size. 



proboscis, and a fin of the shape of an axe-head. This fin is in reality 

 the foot of the snail. The visceral hump is of a brown colour with a sil- 

 very sheen. The animal swims very actively but, curiously enough, with 

 the fin uppermost, the body giving energetic strokes from side to side 

 and the fin swinging to and fro like a pendulum. Their protusible 

 tongue is armed with sharp hooks and with it they catch the lesser pe- 

 lagic animals. The same might be said of Carinaria (Fig. 98), which 

 attains a considerable size and has a delicate transparent shell. 



The Pteropoda are curious, because externally they differ in almost 

 every point from the typical Snails. The head is only indicated by the 

 mouth and the rudimentary tentacles. The body is often covered by a 

 delicate shell. The most striking feature is a pair of large wing-like 

 fins, which are attached to the head or neck and are used by the animals 

 as wings; hence the Neapolitan name farfalle di mare (Sea-butterflies). 

 The most common genus Hyalaea (Fig. 99) has a delicate horny shell 

 of brownish colour, and large fins which are perpetually beating. It 

 appears in swarms, but only lives a day in the Aquarium (Tank 20). 



The oysters, mussels etc. belong to the Bivalves (LamelHbranchia}, 

 the lowest group of the Mollusks. They are distinguished from the 

 snails by their shell, which consists of two pieces which are hinged, and 



Guide. 4th e d. 



