‘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. 
Fie. 153. 
Shell of Cavolinia tridentata, seen 
from the right side. /, postero-dor sal 
surface; g, antero-ventral surface; h, 
median dorsal spine; i, mouth of ‘the 
shell. (From Lankester, after Souleyet.) 
a om 
Ss ig oa, rere 
TRIBE 2. APLYSIOMORPHA. 
oe In these Tectibranchs the shell 
is always much reduced and more 
\ or less internal, or it may be alto- 
a ft gether lost in the adult, eg. in 
u Phyllaplysia and the Gymmnoso- 
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. 
Faminy 1. APLYSIIDAE, 
d’Orbigny. The shell partly 
: covered in, or internal (abgent in 
Clio acicula, ventral aspect. C, 
wing-like lateral lobe of the foot; d, eee ; the foot long, with 
median posterior lobe of the foot; ¢ well - developed ventral surface. 
genital opening ; ; h, pointed extremity 
Fie. 152. 
of the shell; i, anterior margin of the Genera—A plysia, Linnaeus ; shell 
shell ; m, stomach ; 0, liver; p, heart ; i hy j 
U, hermaphrodite ¢l g land. (From Lan- incompletely covered 2 parapodia 
kester, after Souleyet.) broad ; visceral commissure long ; 
Y British (Fig. 154). Dolabella, 
Lamarck. Dolabrifer, Gray. Aplysiclla, Fischer; shell only slightly 
covered in; parapodia slightly developed ; visceral commissure short. 
Phyllaplysia, Fischer ; parapodia slightly developed ; noshell. Notarchus, 
Cuvier ; shell internal, much reduced ; parapodia fused together dorsally 
to form a contractile sac surrounding but not attached to the visceral sac. 
