TUSK-SHELLS. PROTO-MOLLUSCS 



339 



symmetrical, with the upper side concave, and it is covered by 

 a tubular shell shaped something like an elephant's tusk, whence 

 its name. This shell has an aperture at each end, the larger 

 one being in front, and its formation may be understood if we 

 suppose the presence of two long 

 mantle -flaps which have fused 

 together in the middle line below, 

 and that shelly matter has been 

 secreted continuously all round. 

 This view is justified by the develop- 

 ment. So far there would appear 

 to be affinity with the bivalve mol- 

 luscs, and this is confirmed by the 

 shape of the/00/ which, however, 

 has a lobe each side as well as a 

 central portion and by the charac- 

 ter of the nervous system. In other 

 respects there is an approach to 

 Gastropods, for though the head is 

 much reduced it is not entirely 

 absent, the mouth being placed on 

 the end of a short non- re tractile proboscis, behind which is a 

 pharynx provided with the typical rasping- organ. 



Springing from the base of the proboscis are two bunches 

 of long filaments dilated at their ends, and capable of protrusion 

 for some distance from the mouth of the shell. These are sup- 

 posed to be of use in capturing small organisms as food, and it 

 is possible that they represent the gills of other forms, though this 

 is by no means certain. 



Fig. 199. Tusk-shell (Dentalium}. Shell 

 partly removed 



CLASS 5. PROTO-MOLLUSCS (AMPHINEURA) 



The different classes of Molluscs are all supposed to have 

 sprung from bilaterally symmetrical forms with fairly distinct 

 head, a rasping-organ, and a creeping foot. There was probably 

 a continuous mantle-flap sheltering in the hinder-part of the body 

 a pair of plume-like gills, near which would open the paired 

 kidneys, one on each side of the intestinal aperture. The heart 

 would be dorsal and posterior in position, and would most likely 

 consist of a muscular ventricle with a thin-walled auricle on each 



