350 



MOLLUSCA. 



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 there 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 

 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 (Dentaliuni), or carries 

 a retractile disc with papillose 

 margins (Siphonodentalium) ) 

 from the centre of which a 

 filiform tentacle arises in Pul- 

 settum. 



The nervous system con- 

 sists of four groups of ganglia : 

 a pair of cerebral with closely 

 adjacent pleural, a pair of 

 2)edal, and a pair of visceral 

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

 from the pleura!, and the pleuro -pedal commissure is completely 

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

 (stomatogastric) in connection with 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 ; Mh 

 mantle-cavity ; Ffoot ; Mk cephalic prominence 

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

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

 iY kidney ; Ge generative gland. 



