186 INVERTEBRATE ANIMALS. 



developed on the under surface of the body, and known as the 

 "foot." Other Univalves and many Bivalves can effect short leaps 

 by means of the foot ; but many of the latter are permanently fixed 

 to solid objects, or buried in the sand. The minute Mollusca, 

 known as the I'teropoda (fig. 147), swim freely at the surface of the 

 ocean by means of two fins, formed by a modification of the foot, 

 and attached to the sides of the head. The only Molluscs which 

 enjoy really active powers of locomotion are the predaceous Cuttle- 

 fishes, which swim rapidly by means of fins, or by ejecting a jet of 

 water from the cavity of the mantle, and which can also creep about 

 by means of the "arms " placed round the mouth (fig. 148). 



The last feature in the Mollusca which requires to be mentioned 

 is the " shell." The shell is not invariably and universally present 

 in the Mollusca, many being either destitute of a shell altogether, 

 or having one so small that it would not commonly be recognised as 

 such. In these cases, as in the common Slugs, the animal is said to 

 be " naked." In all the Mollusca which possess a shell, this is 

 secreted by the integument, or by what is technically called the 

 " mantle " ; and in all cases it is composed of carbonate of lime. 

 The methods in which the lime is arranged differ in different cases ; 

 but all living shells have an outer covering of animal matter, which 

 is known as the " epidermis." In a great many of the higher Mol- 

 lusiyi, such as the Whelks, Periwinkles, Snails, and others, the shell 

 consists of only a single piece, when it is said to be " univalve." In 

 many others, such as Oysters, Mussels, Scallops, &c., the shell is 

 comjiosed of two pieces, and is then said to be "bivalve." In a few 

 forms the .shell consists of several pieces, and it is then said to be 

 " multivalve.'' The more important variations in the shells of the 

 Mollusca will be noticed in speaking of the different classes of the 

 sub-kingdom. 



In accordance with the nature of the nervous system, the Mol- 

 lusca are divided into two great divisions, known respectively as 

 the Molluscoii.ht. and Molluxio ^o-oper. In the Molluscoida the 

 nervous system consists of a single ganglion, or principal pair of 

 ganglia, and there is either no cii-culatory cirgan or an imperfect 

 heart. In this division are included the three classes of the Sea- 

 mosses (I-'ol//:oo), the Sca-scjuirts {Tu)iii-ato), and the Lamp-shells 

 and their allies (/Jrorhiopoda). In the Molliixea proper the nervous 

 system consists of three princij»l pairs of ganglia, and there is a well- 

 developed heart, consisting of at least two chambers. Under this 

 head come all the ordinary forms of Shell-fish. 



