55 



whelk's radular sac favour, then, the latter theory. The 

 very young teeth are probably entirely formed of chitin. 

 Certainly there is no differentiation given by stains. 

 They very soon become hardened by the deposit of 

 mineral salts, but they differ, as do all other Odontophora, 

 very considerably from the Docoglossa, where the teeth 

 contain a remarkably large proportion of silica. Accord- 

 ing to Miss Sollas, the composition of the teeth of 

 Littorina is : — Ash 3*7 per cent, containing iron, calcium 

 and magnesium, while the rest is organic matter, the 

 basis being chitin. In the Docoglossa, on the other hand, 

 the mineral matter may amount to as much as 27 per cent. 

 (Patella valgata) — silica hydrate being present in large 

 quantity. 



Outside the odontoblast layer is a very compact 

 connective tissue layer, and it is in this connective tissue 

 that the muscles, which are attached to the radula, 

 terminate. The muscles are of the type described above. 

 Their terminations can be followed very beautifully 

 indeed in sections stained in Mallory, for the muscles are 

 bright red, and the connective tissue bright blue. 



BLOOD VASCULAR SYSTEM. 



The vascular system of some molluscs has attained 

 a high degree of complexity. This is especially the case 

 with the Cephalopoda and some of the Prosobranchiata, 

 and the whelk amongst the latter may be taken as showing 

 a good example of a well developed molluscan blood 

 system. It seems that, at the present day, in many 

 zoology courses there is a tendency to pay little 

 attention to the vascular system of the molluscs. 

 The cephalopods receive perhaps adequate treatment, 

 and possibly the snail (Helix) has some attention. 



