MOLLUSCA. 243 



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 pallial line and muscle scars, Fig. 

 107; 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). 



283. 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), (6) rectr actors, 

 which withdraw the whole or special portions of the animal 

 into the shell (lamellibranchs and gasteropods), (c) pro- 

 tractors or extensors, which enable the animal partly to extend 

 itself; (2) pallial (mantle) muscles, best developed in cephalo- 

 pods; (3) the 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 siphon of the same animal is regarded as a modification 

 of a part of the foot. The tentacles about the mouth are also 

 looked upon as arising from the anterior part of the foot, 

 hence the name Cephalopod, which means "head-footed." 



284. Digestive Organs. Mouth and anus both occur, and 

 are usually widely separated. In the coiled forms (as the 

 snail), however, by the looping of the digestive tract they are 

 brought close together. In all except the group of headless 

 mollusks (lamellibranchs) the mouth is supplied with a radula, 

 or tooth-bearing tongue. This lies in the floor of the mouth 

 and, as it is worn away in front, is renewed from behind in 



