GENERAL CHARACTERS. 381 
sisting typically of (a) a loop connecting the two pleurals and 
provided with two visceral ganglia, and (b) a stomato-gastrie 
ope f zl J? gab, 
Fic, 205.—Ideal mollusc.—After Ray Lankester. 
m., Mouth ; g.c., cerebral ganglia; c., edges of mantle skirt; z.g., 
duct of right lobe of digestive gland ; s., pericardial cavity ; /, 
edges of shell-sac; w., ventricle of heart; %., nephridium 3 az., 
anus ; #., posterior part of the foot ; 2., opening of nephridium ; 
&., genital aperture; g.ad., abdominal ganglion on visceral 
loop ; g.v., visceral ganglion ; 2.2., left lobe of digestive gland ; 
B., foot ; g.fe., pedal ganglion; g.A/. pleural ganglion. 
loop connecting the cerebrals below the gullet and provided 
with two buccal ganglia (Fig. 205). Lxcept in Lamelli- 
branchs, in which the head region is degenerate, there is in the 
Fic. 206.—Stages in molluscan development. 
D, Larva of Heteropod (after Gegenbaur); sk., shell covering 
visceral hump; v., velum; 7, foot. 
E, Larva of Atlanta (after Gegenbaur); v., velum; s%., shell; 
J, foot ; of., operculum. 
mouth a chitinous ritbon or radula, usually bearing numerous 
small teeth, and moved by special muscles, the whole structure - 
being known as the odontophore. There is much unstriped 
muscle, but the more rapidly contracting muscles have cross- 
