350 



MOLLUSCA. 



S 



The body is elongated, slightly curved, with a dorsal concavity. 

 The shell is shaped like an elephant's tusk, and is open at both ends. 

 The animal, which is entirely contained within ' the shell, has a 

 similar shape, and is enclosed by a tubular mantle also open at 

 the two ends. It is attached to the shell by a muscle at the 

 hind end. The mantle is to be regarded as paired folds of the 

 dorso- lateral integument which have undergone concrescence ven- 

 trally. 



A distinct head is not present, but tnere is an egg-shaped projection 

 into the mantle-cavity at the front end, at the apex of which is the 



mouth, surrounded by leaf -like 

 labial appendages. Springing 

 from two lobes at the base of 

 the cephalic prominence are a 

 number of ciliated contractile 

 filaments (captacula) which 

 are swollen at their ends. 

 These have been supposed to 

 ~Mh represent the ctenidia, which 

 are otherwise not represented. 

 The foot is cylindrical and 

 directed forwards; it can be 

 protruded through the ante- 

 rior (larger) opening of the 

 mantle. Its front end is tri- 

 lobed (Dentalium\ or carries 

 a retractile disc with papillose 

 margins (Siphonodentalium), 

 from the centre of which a 

 filiform tentacle arises in Pul- 

 sellum. 



The nervous system con- 

 sists of four groups of ganglia : 

 a pair of cerebral with closely 

 adjacent pleural, a pair of 

 pedal, and a pair of visceral 

 ganglia just in front of the anus. The visceral commissure arises 

 from the pleural, and the pleuro- pedal commissure is completely 

 fused with the cerebro-pedal. There is a system of labial ganglia 

 (stomatogastric) in connection w r ith the buccal mass : it is connected 

 with the cerebral. 



Mt 



FIG. 273. Dentalium as seen in longitudinal 

 section (except the foot) after Grobben. S 

 shell ; Mt mantle ; Sm shell muscle ; M h 

 mantle-cavity ; F foot ; Mk cephalic prominence 

 or oral cone ; T captacula ; R radula ; D intes- 

 tine ; L liver ; Af anus ; G cerebral ganglion ; 

 N kidney ; Ge generative gland. 



