GENERAL INTRODUCTORY n 



further contain as much as 27 per cent, of silica 

 hydrate or opal in their composition, while in the 

 rest of the Gastropoda the chitine is hardened super- 

 ficially by deposits containing calcium, iron, and 

 phosphoric acid, amounting in all from 2*4 to 6 per 

 cent. The Chitons differ from this second group in 

 alone having ferric oxide as the most important 

 mineral constituent. In use, not only is the whole 

 tongue-like mass protruded and worked with a 

 licking action, but the radula moves backwards and 

 forwards on its surface over the cartilages like a 

 chain saw. The number of teeth in each transverse 

 row varies from one, as in certain of the sea-slugs 

 {Molis, Elysia, etc.) and species of Chcetoderma, to 

 upwards of 200 or 300, as in the Top Shells and 

 their allies (Trochus, Haliotis, etc.). When more 

 than one is present the teeth generally vary in shape, 

 but those on either side of the centre correspond 

 each to each. The number of rows also varies from 

 only a few to very many, so that in some Mollusca, 

 like the Limpet, the radula is nearly twice as long 

 as the animal. 



In all, the teeth in front tend to become worn 

 away by constant use, and to replace them fresh 

 teeth are continually being formed at the other end, 

 which is kept in a special sac or pouch under the 

 gullet. The longest radulse are generally found in 

 those molluscs, like the Limpet, that feed on the 

 microscopic plants growing on rocks, the teeth in 



