MOLLUSC A 



209 



invertebrate type, but, as in the eyes of vertebrates, the 

 nerve pierces the layer of sensory cells, and is distributed 

 to that side of them which is nearest the lens. In Onchidium, 

 as in Pecten, etc., the eyes are probably modifications of certain 

 simple tentacles which are borne on the mantle. 



Class Scaphopoda. 



Chaeacteeistics. — Elongated cylindrical Glossofhora. The 

 mantle has extended on to the ventral surface, and has fused in 

 the middle ventral line. It has secreted around it a cylindri- 

 cal shell. The foot can he protruded through the anterior 

 opening of the shell. No heart is present. The visceral loop 

 of the nervous system is untwisted. Dioecious, the generative 

 products escape through the right nephridium. 

 This class includes but three genera, of which the best- 

 known is Dentalium. The cylindrical shape of its body 

 may be correlated with a burrowing habit of life, and it is 

 interesting to note that a similar shape is found amongst 



Fig. 123. — Diagram of the ana- 

 tomy of DentcUmm. Lateral 

 view of organs, showing as 

 though by transparency. After 

 Lankester — Zoological Articles 

 reprinted from the JSncyclo- 

 pcedia Britannica. 



10. Peri-oral part of mantle cavity. 



11. Foot. 



12. Cerebral ganglion. 



13. Pleural ganglion. 



14. Olfactory ganglion placed on visceral 



loop as in the Lamellibranchiata, 

 according to Spengel. 



15. Pedal ganglion. 



16. Left nephridium. 



1. Mouth surrounded by pinnate ten- 



tacles. 



2. Oral process. 



3. The ctenidial filaments. 



4. Odontophore. 



5. Oesophagus. 



6. Left lobe of liver. 



7. Anus. 



8. Peri-anal part of mantle cavity. 



9. Appendix of mantle skirt separated 



by a valve from 8. 



the sand -boring Lamellibranchiata, such as Solen. The 

 mantle is ensheathed by a shell, which is at first incom- 

 plete ventraUy. The shell resembles a truncated elephant's 

 tusk, it is open at both ends, the larger opening is the 

 anterior one, and the concave surface is dorsal. 



The foot is protrusible through the anterior opening of 



14 



