248 ZOOLOGY 



characteristic of MoUusca. It grows out from the dorsal wall 

 of the body and encloses a space known as the mantle cavity. 

 It is usually important in respiration, and contains the shell- 

 glands. 



288. Shells are formed in all the classes of Mollusca, al- 

 though naked forms occur in several of them. The shell is a 

 true secretion or excretion, deposited by the epithelial layer of 

 the mantle. It consists of three layers: (a) a thin external 

 layer of organic material known as conchiolin, (b) the prismatic 

 layer, embracing the greater thickness of the shell and made up 

 of prisms of carbonate of lime cemented by conchiolin, and (c) 

 the nacreous or pearly layer over the inner surface. The edge 

 of the mantle secretes the first and second layers, and they 

 usually show lines of growth parallel with the edge of the mantle; 

 the pearly layer is deposited by the whole surface of the mantle. 

 The point of attachment of the muscles presents a depression in 

 this layer because the deposit has been interrupted (see palHal 

 line and muscle scars. Fig. 109; and in shell of clam). 



In some Cephalopods there is an internal skeleton in part 

 secreted by the mantle (cuttle bone), and in part formed of 

 cartilage (the brain case). 



289. The muscular system is made up of unstriped muscle 

 fibres, which usually occur in more or less prominent bands or 

 muscles. These may be classified as follows: (i) shell or skeletal 

 muscles, which embrace (a) adductors, those which draw the 

 valves together (lamellibranchs), (&) retractors, which withdraw 

 the whole or special portions of the animal into the shell (lamel- 

 libranchs and gasteropods) , (c) protractors, or extensors, which 

 enable the animal partly to extend itself; (2) pallial (mantle) 

 muscles, best developed in cephalopods; (3) tine foot, which is a 

 mass of muscle and is one of the most characteristic of the 

 molluscan organs ; and (4) minor muscles controlling the radula 

 or tongue, the other mouth parts, and the like. 



Locomotion in the group is accomplished chiefly by the foot, 

 in its various modifications, or by rhythmic opening and shut- 

 ting of the valves. The squid has a fin-like extension of the 

 integument which is an efficient organ of forward motion. The 



