434 



PHYLUM MOLLUSCA 



sisting typically of (a) a loop connecting the two pleurals and 

 provided with tvjo visceral ganglia, and (b) a stomato-gastric 

 loop connecting the cerebrals below the gullet and provided 

 with two buccal ganglia (Fig. 247). Except in Lamelli- 

 branchs, in which the head region is degenerate, there is in the 

 mouth a chitinous ribbon 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- 

 striped fibres, or fibres with unstriped fibrils twisted in a 



Fig. 247. — 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 ; i;., ventricle of heart ; j<., nephridium ; an., 

 anus ; n., posterior part of the foot ; /., opening of osphradiiim ; 

 k., genital aperture ; g.ab., abdominal ganglion on visceral 

 loop ; g.v., visceral ganglion ; z.l., left lobe of digestive gland ; 

 p. foot ; g.pe., pedal ganglion ; g.pL, pleural ganglion. 



Spiral. A portion of the true body cavity or coelom usually 

 persists as the pericardium at least (Fig. 247, s.), and 

 communicates with the exterior through the nephridium or 

 nephridia. The rest of the cavity of the body is hcemocoelic. 

 The vascular system is almost always well developed, but 

 part of the circulation is in most cases lacunar ; the heart 

 typically consists of a ventricle and two auricles. Respiratory 

 organs are most typically represented by gills or ctenidia, 

 consisting of an axis attached to the body and bearing lamella, 

 but the gills may have simpler forms, or may be absent, and 

 in the terrestrial snails the mantle cavity is adapted for 

 aerial respiration. At the base of the gills there is generally 



