MOLLUSCS AND BRACHIOPODS. 



443 



and family, the radula is characteristic ; thus, in Melongena vesper- 

 tilio (Fig. 3), the central tooth is tricuspid, the central cusp being 

 the smallest, while the laterals are bicuspid ; in Ebtirna Japofika 

 (Fig. 4) the central tooth is 5-cusped, the two outer cusps being 

 much the smallest. The teeth, on the whole, are sharp and hooked, 

 with a broad base and formidable cutting edge. 



Fig. 5. — View of the left half of 

 Cistella ( Argiope) neapolitana, which 

 has been cut in two by a median longi- 

 tudinal incision to show the disposition 

 of the organs. Partly diagrammatic. 

 The inorganic part of the shell only is 

 shown. The tubular extensions of the 

 mantle and the organic outer layer are 

 not included, and hence the pores 

 appear open. 



I. — The ventral valve. 



2. — The dorsal valve. 



3. — The stalk. 



4. — The mouth. 



5. — Lip, wMch overhangs the mouth 

 and runs all round the tentacular arms. 



6. — Tentacles. 



7. — Ovary in dorsal valve. 



8. — Liver diverticula. 



9. — Occlusor muscle ; its double or- 

 igin is shown. 



10. — Internal opening of left neph- 

 ridium. 



II. — External opening of left neph- 

 ridium. 



12. — Ventral adjustor. The line from 

 10 crosses the dorsal adjustor. 



13. — Divaricator muscle. 



We will now turn to Chapter XVII., which treats of Recent 

 Brachiopoda. The body of a Brachiopod is enclosed within a 

 bivalve shell, but the two halves are not, as they are in the Pele- 

 cypoda, one on each side of the body, but occupy a different posi- 

 tion with regard to the main axis of the body (Fig. 5). 



The shell of a Brachiopod is secreted partly by the general 



