GASTEROPODA 569 



Cavolinia (Fig. 384 D) is an example of the Pteropoda (sea butter- 

 flies), a special group of the Opisthobranchiata which are modified for 

 pelagic life. They usually have a transparent uncoiled shell in the 

 form of a quiver or a vase, from the aperture of which projects the 

 foot in the form of two fins, the epipodia. By the slow flapping move- 

 ment of these the pteropods progress through the water. There are 

 ciliated tracts on the fins, and by the action of the cilia on these, small 

 organisms are sifted from the water and collected in the mouth, the 

 radula assisting in swallowing. Limacina is a pteropod with a coiled 

 shell. 



Eolis (Fig. 384 C) is a nudibranch which possesses a series of dorsal 

 processes (the cerata)^ which contain diverticula of the digestive gland, 

 each of w'hich opens to the exterior at the tip of the process. The 

 animal feeds on hydroids or sea anemones, and while most of the food 

 is digested or passes out of the anus, the nematocysts are collected in 

 terminal sacs in the cerata and when the animal is irritated they are 

 ejected and everted. This is a unique example of the use in defence 

 by one animal of the oflPensive weapons of another. The cerata are 

 often brilliantly coloured and experiments with fish show that sea 

 slugs are avoided on account of their "warning" patterns. 



Hermaea is another nudibranch with similar cerata, which have not, 

 however, openings to the exterior. The animal feeds on green algae 

 (Siphonales). The radula, in each row of which there is only a single 

 sharp tooth, forms a saw by which the cell wall of the alga is opened. 

 Then by dilatation of the buccal cavity the fluid protoplasm is sucked 

 out. 



Doris (Fig. 384 B), the sea lemon, a short flattened nudibranch, 

 sluggish in movement, which feeds on incrusting organisms like 

 sponges. There is a tough mantle, which is usually pigmented and 

 often resembles the feeding ground, and is reinforced by calcareous 

 spicules. Anteriorly there is a single pair of short tentacles and 

 posteriorly a median anus surrounded by a tuft of accessory gills. In 

 front of the anus is the median kidney aperture. The nervous system 

 is centralized round the oesophagus, and the generative aperture 

 occurring on the right side is the only external organ which is 

 asymmetrical. 



Order PULMONATA 



Hermaphrodite gasteropods, most of which exhibit torsion and have 

 a shell (but no operculum), but which have a symmetrical nervous 

 system, the symmetry being due to the shortening of the visceral 

 connectives and the concentration of the ganglia in the circum- 

 oesophageal mass; with a mantle cavity which has become a lung, 

 without a ctenidium, but with a vascular roof and a small aperture 



