SCAP1IOPODA. 



29 



-Br 





(fig. 506). The body possesses a saccular mantle, like the shell open 

 at both ends, and a trilobed foot ; the foot is protruded through the 

 larger of the openings of the shell- from the anterior opening of the 

 mantle, the margin of which is thickened. A separated cephalic 

 region is not present, .but there is an egg-shaped projection in the 

 mantle cavity, at the apex of which is placed 

 the mouth, surrounded by eight leaf-like labial 



appendages. 



The buccal armature con- 

 sists of a lateral (right and 



left) rudimentary jaw, and a 



tongue beset with five rows of 



plates. 



The alimentary canal is 



divided into a buccal cavity, 



resophagus, stomach with large 



liver, and an intestine, which 



after several coils closely 



pressed together, opens behind 



the foot into the middle of the 



mantle cavity. 



The circulatory organs are 



reduced to two mantle vessels 



and a complicated system of 



wall-less spaces of the body 



cavity. 



Respiration is effected by 



the surface of the mantle and 



also by the filiform tentacles, 



which arise from two ridges 



(cervical collar) behind the 



head-like buccal prolongation. 

 The kidney lies round the 

 rectum, and opens by two openings placed on 

 the right and left of the anus. 



The nervous system consists of three groups 

 of ganglia, of which the pedal ganglion bears two otocysts. Eyes are 

 absent. The numerous ciliated tentacles serve as tactile organs. 



The Sca/pkopoda are dioecious. The ovaries and testes are un- 

 paired finger-shaped lobed glands, which are placed behind the liver 

 and intestine, and open to the exterior with the right kidney. 



FIG. 50C. 



Tare nt in urn (after 

 Lacaze- Duthiers). 

 Animal without 

 ' shell from right 

 side. .P.foot; Mt, 

 circular muscle of 

 mantle ; M, longi. 

 tudinal muscle ; 

 BI-, gills ; N, kid- 

 ney; L, liver; G, 

 generative gland. 



s 



FIG.. 507. Larva of Dentu- 

 Hi'.m (after Lacaze-Du- 

 thiers). a, young larva 

 with first rudiment of 

 shell (S). I, Older larva 

 seen from the dorsal sur- 

 face ; T, tentacle collar ; 

 Gff, cerebral ganglion ; 

 Oet, oesophagus ; L, liver. 



