494 THE INVERTEBRATA 



The opisthobranchs are classified as follows : 



Tectibranchiata. Opisthobranchiata which often have a shell 

 and nearly always a mantle cavity and ctenidium. Actaeon, Bulla^ 

 Aplysia, Cavolinia. 



NuDiBRANCHiATA. Opisthobranchiata usually of slug-like habit 

 which have neither a shell, nor a mantle cavity, nor a ctenidium. 

 Eolis^ Doris. 



Aplysia (Fig. 364 A), the sea hare, is found crawling on seaweeds 

 which form its food. The younger forms occur in rather deeper water 

 and are red in colour, matching the red algae on which they occur, 

 while the larger individuals, between tidemarks, devour green sea- 

 weeds such as Ulva and are olive-green. The head possesses two pairs 

 of tentacles, the anterior being large and ear-like (hence the animal's 

 name), while those of the second pair are olfactory in function and 

 have each a simple eye at their base. From the sides of the foot in 

 the posterior region rise two upwardly directed flaps, the parapodia: 

 by using these the animal can swim. The mantle is reflected over the 

 shell so as to cover all except a small area and the mantle cavity lies to 

 the right of this with the ctenidium pointing backwards, while the 

 anus is at the posterior end. In the walls of the mantle cavity are 

 unicellular glands which secrete the purple pigment ejected by the 

 animal when it is molested. There is a single generative aperture and 

 a single duct for the sperm and ova but a seminal groove runs forward 

 from the aperture to the head and reciprocal fertilization is possible. 

 The only internal characters which need be mentioned are the 

 nervous system, with its well-developed but perfectly symmetrical 

 visceral loop, and the alimentary canal which, in front of the stomach, 

 is dilated into a crop, lined with horny plates, in which the seaweed 

 is masticated before digestion. 



Cavolinia (Fig. 364 D) is an example of the Pteropoda (sea butter- 

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

 pelagic life. They 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 movement 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. 



Eolis (Fig. 364 C) is a nudib ranch which possesses a series of dorsal 

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

 each of which 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 



