MOLL USC A. 413 



pieces, bivalve. The bivalve Mollusca exhibit no traces of 

 any distinct head; whilst, in the univalve, this part of the 

 body is well-marked, and usually furnished with special 

 organs of sense (tentacles, eyes, &c.). 



Fig. 206. Limax rufus, the Common Slug. 



The older naturalists also recognised a group of multi- 

 valve shells, or shells composed of several valves. The 

 majority of which belonged to the Cirrhopod order of 

 Crustacea, and were regarded as Mollusca by earlier ob- 

 servers. The Pholades, however, which in other respects 

 are true bivalve Mollusca, are furnished with a pair of 

 accessory plates in the neighbourhood of the hinge ; whilst 

 the Chitons, a small but singular group of Molluscs nearly 

 allied to the univalve limpets, have an oval shell composed 

 of eight moveable plates, which gives them a great resem- 

 blance to enormous woodlice ; and they have been regarded 

 as forming a sort of transition towards the articulated divi- 

 sion. Some Mollusca are not furnished with a shell, or have 

 only a small calcareous plate enclosed within the mantle. 

 These are called naked Mollusca; 

 an example of this family is seen 

 in fig. 207, Aplysia; but it is re- 

 markable, that most of them are 

 provided with a small shell when 

 they first quit the egg. In the shell- 

 bearing or testaceous Mollusca, this 

 embryonic shell, which often differs 

 greatly in shape and texture from 

 the shell of the mature animal, 

 nevertheless is a commencement of the latter, additions 



Fig. 207. Aplysia. 

 hare. 



Sea- 



