Tank No. 20. 
57 
then the other, the body helping with violent writhings. They live only 
a few weeks in captivity. 
Equally remarkable but of smaller 
size is Aeclis (Fig. 93, Tank 20). 
Doris also (Fig. 94) with its dorsal 
tufts of feathery gills belongs to this 
group. Frequently they may be ob¬ 
served laying eggs, which are generally 
enclosed in a. mucilaginous mass and Fig 94 J)wii tuherculata ^ 
arranged in ribands or strings. i/ 2 nat. size. 
Allies of the snails form a portion of the pelagic fauna (see Note 
on p. 26). 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, w r hen the sea is full of 
pelagic 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> 96. Carinaria mediterranean ^2 nat. size. 
(Fig. 95), a perfectly transparent, long and thin animal with a curved 
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 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 
