CHAPTER III. 



MOLLUSCA. 



: HE term Mollusc is derived from the Latin, and 

 signifies soft ; the body of the animal being soft and 

 fleshy, partly or entirely covered by a shell attached 

 to it by means of muscles. The shells are of two kinds ; 

 those of an epidermal character being formed upon the sur- 

 face of a filmy cloak-like organ called a mantle, answering 

 to the true skin of other animals ; and those of a dermal 

 character being concealed within the substance of the man- 

 tle, and frequently moulded into a great diversity of forms, 

 and coloured with various tints. 



The animals belonging to the molluscous sub-kingdom 

 are divided into the following orders, viz. Bryozoa, Tuni- 

 cata, Conchifera, Gasteropoda, Pteropoda, and Cephalopoda; 

 of these, all, except the Tunicata and a few of the Pteropoda, 

 are provided with a hard calcareous shell. In the first 

 class of the Mollusca, the Bryozoa approximating so closely 

 to Zoophytes, and from their having been until lately 

 classified with them, we have thought it more convenient 

 to the microscopist to retain them in that class with the Eschara. 



In the Conchifera the most simple rudimentary form of shell is met 

 with j for example, in the common slug, Limax rufus, it occurs as a thin 

 oval plate, imbedded in the shield situated on the back and near the 

 head of the animal. The shell of all the oyster genus (Pinna) is com- 

 posed of a series of hexagonal cells filled with transparent calcareous 

 matter, seen at fig. 7, Plate VI. Dr. Carpenter has shown that the 

 outer layer of the shell can be split up into prisms, like so many 

 basaltic columns, fig. 93, No. 1. 



When molluscous shells are composed of a single piece, they are 

 termed univalves ; when of two pieces, bivalves. The bivalve Mollusca 

 exhibit no traces of any distinct head ; whilst, in the univalves, this 

 part of the body is well-marked, and usually furnished with special 

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



The older naturalists also recognised a group of multivalve shells, or 



