GENERAL INTRODUCTORY II 
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, arnounting 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 
(4olis, Elysia, etc.) and species of Chetoderma, to 
upwards of 200 or 300, as in the Top Shells and 
their allies (T’vrochus, 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 radule are generally found in 
those molluscs, like the Limpet, that feed on the 
microscopic plants growing on rocks, the teeth in 
