186 IN'VEKTEISKATE AXIMALS. 



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 i)ermanently fixed 

 to solid objects, or Ijuried in tlie sand. The minute JfoUiisea, 

 known as tlie l'ierop<iih-i (tig. 147', swim freely at the surface of the 

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

 and attaclied to the sides of the head. The only Mollu.scs which 

 enjoy really active [lowers of locomotion are the predaceous Cuttle- 

 fishes, wdiich swim ra]iidly 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 " ]>laced round the mouth (fig. 148). 



The last feature in the Mnlluxi-a which recjuires to be mentioned 

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

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

 or having one so small that it wovdd not connnonly be recognised as 

 such. In these cases, as in the conunon Slugs, the animal is said t(j 

 be "naked." In all the JJn/hixrto which jjossess a shell, this is 

 .secreted Ijy the integument, or bj' wdiat is technically called the 

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

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

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

 is known as the " e|iidermi.s." In a great many of the higlier J/o/- 

 htsca, .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 other.s, such as Oysters, Mus-sels, Scallops, &c., the shell is 

 com]iosed of two pieces, and is then said to lie "bivalve." In a few 

 forms the sliell consi.sts of several jiieces, and it is then said to be 

 " multivalve." The more im|iortant variations in the shells of the 

 iliMuscd will be niiticcil in sjieaking of the dittVrent cla.sses of the 

 sub-kingdom. 



In accordance with the nature of the nervous system, the d/o/- 

 ?«.«('(( are diviiled into tw(j great divisions, known res|iectively as 

 the MnUiisfiiiihi and MnlhisrK projin-. In the J/olliisroula the 

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

 ganglia, ami there is either no circidatory organ or an imperfect 

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

 mosses {/'o/j/:o:i), the Sea-squirts (Tii iiij-irt,i\ ;i\\i\ the Lamp-shells 

 .■uid their allies (Br<i<:hiiiji(>ihi). In the ^[l>Ullxl■a prnprr the nervous 

 .system consists of three )iriiici|ial |)airs of ganglia, and there is a well- 

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

 head come all the ordinary forms of Shell-fish. 



