THE GASTROPODA 



171 



certain species of Peraclis and Cavolinia ; and by the position of the nerve- 

 centres at the sides and on the ventral side of the oesophagus. All three 

 families are pelagic. 



FIG. 153. 



Shell of Cavolinia tridentata, seen 

 from the right side. /, postero - dorsal 

 surface ; g, antero-ventral surface ; h, 

 median dorsal spine ; i, mouth of the 

 shell. (From Lankester, after Souleyet.) 



TRIBE 2. APLYSIOMORPHA. 



In these Tectibranchs the shell 

 is always much reduced and more 

 or less internal, or it may be alto- 

 gether lost in the adult, e.g. in 

 Phylkiplysia and the Gymnoso- 

 mata. The head bears two pairs 

 of tentacles. The margins of the 

 foot, or parapodia, are separate 

 from the ventral surface and are 

 generally transformed into natatory 

 lobes (Fig. 155, fi). The visceral 

 commissure, except in Aplysia, is 

 very much shortened. The genital 

 duct is monaulic ; the hermaphro- 

 dite duct is connected with the 

 penis by a ciliated groove (Fig. 

 154). The animals comprised in 

 this tribe are crawling or swim- 

 ming forms. 



FAMILY 1. APLYSIIDAE, 

 d'Orbigny. The shell partly 

 covered in, or internal (absent in 

 Aplysiella) ; the foot long, with 

 well - developed ventral surface. 

 Genera Aplysia, Linnaeus ; shell 

 incompletely covered ; parapodia 

 broad ; visceral commissure long ; 

 British (Fig. 154). Dolabella, 

 Lamarck. Dolabrifer, Gray. Aplysiella, Fischer; shell only slightly 

 covered in ; parapodia slightly developed ; visceral commissure short. 

 Phyllaplysia, Fischer ; parapodia slightly developed ; no shell. Notarchus, 

 Cuvier ; shell internal, much reduced ; parapodia fused together dorsally 

 to form a contractile sac surrounding but not attached to the visceral sac. 



FIG. 152. 



Clio acicula, ventral aspect. C, 

 wing-like lateral lobe of the foot ; d, 

 median posterior lobe of the foot ; c, 

 genital opening ; h, pointed extremity 

 of the shell ; i, anterior margin of the 

 shell ; n, stomach ; o, liver ; p, heart ; 

 u, hermaphrodite gland. (From Lan- 

 kester, after Bouleyet.) 



