100 IV. 5. 



severally. In the Sea-snails, for example, we find teeth, hard and. 

 sharp, as before mentioned,^^ and between them a fleshy substance 

 like the tongue of Crustacea and Cephalopods, and again a 

 proboscis which has already been described as something between 

 a piercing instrument and a tongue." Directly after the niouth 

 comes a kind of bird-like crop,^® then a gullet, and continuous 

 with this a stomach, in which is the mecon^ as it is styled ; and this 

 mecon in turn gives rise to an intestine, starting directly from it.^^ 

 It is the excretion of this mecon, which appears in all the Testacea 

 to form the most palatable morsel. Purpurae,^° and whelks, and 

 all other Testacea that have turbinated shells, in structure 

 resemble the sea-snail. 



The genera and species of Testacea are very numerous. There 

 are those with turbinated shells, of which some have just been 

 mentioned ; and, besides these, there are bivalves and univalves. 

 Those with turbinated shells may indeed after a certain fashion be 

 said to resemble bivalves. For they all, from their very birth, 

 have a covering to protect that part of their body which would 

 otherwise be exposed.^^ This is the case with the Purpurae, with 

 whelks, with the Nerites,^^ and all the like. Were it not for this, 

 the part which is undefended by the shell would be very liable to 

 injury from without. The Univalves also are not without pro- 

 tection. For on their dorsal surface they have a shell, and by 

 the under surface they attach themselves to the rocks, and so after 

 a manner may be said to become bivalved, the rock representing 

 the second valve. Of these the animals known as limpets are an 

 example. The Bivalves, scallops and mussels for instance, are 

 protected by the power they have of closing their valves ; and the 

 turbinated genera by the operculum just mentioned, which trans- 

 forms them, as it were, from univalves into bivalves. But of all 

 there is none so perfectly protected as the Echinus. For here 

 there is a globular shell which encloses the body completely, 

 and which is moreover set with sharp spines. This peculiarity 

 distinguishes the Echinus from all other Testacea, as has already 

 been mentioned. 



The structure of the Testacea and of the Crustacea is exactly the 

 reverse of that of the Cephalopods. For in the latter the fleshy 

 substance is on the outside and the earthy substance within, whereas 

 in the former the soft parts are inside and the hard part without. 

 In the Echinus, however, there is no fleshy part whatsoever. 

 679b. 



