GASTEROPODA 567 



cardium compressed into a small visceral hump. The animal swims 

 ventral surface uppermost, using its foot as a fin. The sucker is a 

 rudiment of the crawling surface. It is predaceous, seizing worms 

 and other animals with its radula and swallowing them whole. 



Order OPISTHOBRANCHI ATA 



Hermaphrodite gasteropods which are descended from Streptoneura 

 which have undergone torsion but themselves show a reversal of 

 torsion (detorsion); with the mantle cavity, where present, tending to 

 occupy a posterior position again, the shell to become smaller, in- 

 ternal or entirely absent and the single ctenidium to disappear and 

 be replaced by accessory respiratory organs or by the whole external 

 surface becoming a respiratory organ. 



The opisthobranchs are classified as follows: 



Tectibranchiata. Opisthobranchiata which often have a shell 

 and nearly always a mantle cavity and ctenidium. Actaeon, Bullae 

 Aplysia, Cavolinia. 



NuDiBRANCHiATA. Opisthobranchiata usually of slug-like habit 

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

 EoliSy Doris. 



Aplysia (Fig. 389 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 impossible. 

 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. 



