SHELL OP MOLLUSCA, 529 



summer and towards autumn, often obtaining to a consi- 

 derable size." Most of the species may be found in the 

 ponds around London, and few objects are capable of 

 affording greater pleasure than these Polyzoawhen examined 

 in a living state under a moderate power, and with a dark- 

 ground illuminator. The withdrawal of the Polyzoa 

 from the Eadiate sub-kingdom, and their location among 

 Mollusca, was a step in the right direction ; while the 

 important division of the molluscan sub-kingdom by 

 Milne-Echvards into primary sections of the Mollusca and 

 Molluscoida, the latter including the Tunicata and the 

 Polyzoa, is all that can be desired in the systematic 

 location of the Polyzoa. 



Shell of Mollusca. — The simplest form of shell occurs in 

 the rudimentary oval plate of the common Slug, Limax 

 rufus ; it is imbedded in the shield situated at the back 

 and near the head of the animal. 



When a molluscous or conchiferous shell is composed of 

 a single piece, it is then termed univalve ; when of two 

 pieces, bivalve. The bivalve Mollusca exhibit no trace 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, nerves, &c). 



The older naturalists recognised a group of niulti- 

 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 Plbolades, 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 sin- 

 gular group of Molluscs nearly 

 alHed to the univalve limpets, have 

 an oval shell composed of eight 

 movable plates, which gives them a 

 great resemblance to enormous 

 woodlice ; and they have been re- 

 garded as forming a sort of transi- 

 tion towards the articulated divi- Fig. 21s.— Apiysia, ses- 

 sion. Those Mollusca not furnished 

 with a shell, or having only a small calcareous plate en- 



M M 



