Mollusca. 



95 



a short time in the Aquarium. Tliey swim by throwing the head 

 first to one side and then to the other, the body helping by violent 

 writhings. The stomach is often full of young fishes. 



Equally beautiful in colour but of smaller size is Aeolis (fig. loi). 

 Doris also (fig. lOo) 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. 



Snails too are present among the pelagic animals (see p. 57). 

 Swimming among the transparent Medusae and Ctenophora, we 

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

 ■poda). Especially in spring and autumn, when 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. 97), 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 vis- 

 ceral hump is of a brown colour with a silvery 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 

 pelagic animals, and even eat one another. The same is true of 

 Carinaria (fig. 98) which is distinguished from Pterotrachea by 

 the possession of a small transparent shell. This animal is even 

 capable of digesting the stinging organs oiPhysalia, a large Siphono- 

 phore, the sting-cells of which are deadly to much larger animals. 



Tlie Pteropoda are curious because externally they differ in 

 almost every point from the typical snails. Tne 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 

 Hyalea (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 x\quarium (tank 20). 



The oysters, mussels, etc. belong to the Bivalves {Lamelli- 

 hranchia), the lowest group of the Mollusks. They are distinguished 

 from the snails by their shell, which consists of two pieces which 

 are hinged, and are brought together by means of one or two 

 muscles, but open by an elastic external ligament when the muscles 

 are relaxed. The absence of a head is a characteristic feature 

 of this group. The protrusible "foot" (fig. 165 on the left) serves 

 as organ of locomotion. The body is covered on both sides by 

 the leaf-like gills, and the latter by the two mantle-flaps which 



