PARTS OF ANIMALS, IV. v. 



Purpuras and the Whelks) are similar to the Sea- 

 snails in structure. 



There are very many genera and species of Tes- 

 tacea. Some have spiral shells, like the ones just 

 mentioned ; some are bivalves, some univalves. In 

 a way, the spiral shells resemble the bivalves, as they 

 have, all of them, from birth, a covering ° over the 

 exposed part of their flesh, e.g. the Purpuras, the 

 Whelks, the Nerites, and the whole tribe of them. 

 This covering serves as a protection ; for in any place 

 where the animal has no shell to protect it, it could 

 quite easily be injured by the impact of external 

 objects. The univalves' means of preservation is this : 

 they cling to some object, and have their shell on the 

 upper side ; so they become in a way bivalves in 

 virtue of the borrowed protection afforded by the 

 object to which they cling. Example, the Limpets. 

 The bivalves proper (e.g. Scallops and Mussels) get 

 their protection by closing themselves up ; the spiral- 

 shelled creatures by the covering I mentioned, which, 

 as it were, turns them from univalves into bivalves. 

 The Sea-urchin has a better defence system than any 

 of them : he has a good thick shell all round him, 

 fortified with a palisade of spines. As I stated pre- 

 viously, the Sea-urchin is the only one of the Testacea 

 which possesses this peculiarity. 



The natural structure of the Crustacea and of the 

 Testacea is the reverse of that of the Cephalopods. 

 The latter have their fleshy part outside, the form.er 

 have the earthy part outside and the fleshy inside. 

 The Sea-urchin, however, has no fleshy part at all. 



All these parts, as described — mouth, tongue-like 

 object, stomach, vent for the residue — are present 

 in the rest of the Testacea too, but they differ in 



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