MOLLUSCA 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. 
CHARACTERISTICS.—Llongated cylindrical Gllossophora. The 
mantle has extended onto the ventral surface,and has fused in 
the middle ventral line. It has secreted around it a eylindri- 
cal shell. The foot can be protruded through the anterior 
opening of the shell. No heart is present. The visceral loop 
of the nervous system is unturisted.  Duoecious, 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 
BES sox (8 sj Fic. 123.—Diagram of the ana- 
RR tomy of Dentaliwm. Lateral 
view of organs, showing as 
though by transparency. After 
Lankester—Zoological Articles 
atoms = 8 reprinted from the Hncyclo- 
i ‘ / ' ‘ . . . 
ot / p ' cedia Britannica. 
TE 15 10 G10: 7 a 
1. Mouth surrounded by pinnate ten- 10. Peri-oral part of mantle cavity. 
tacles. 11. Foot. 
2. Oral process. 12. Cerebral ganglion. 
3. The ctenidial filaments. 13. Pleural ganglion. 
4, Odontophore. 14. Olfactory ganglion placed on visceral 
5. Oesophagus. loop as in the Lamellibranchiata, 
6. Left lobe of liver. according to Spengel. 
7. Anus. 15. Pedal ganglion. 
8. Peri-anal part of mantle cavity. 16. Left nephridium. 
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 ventrally. 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 
