170 THE ANIMALS AND MAN 



peculiarly their own. In them the body is not articulated or 

 segmented as with the worms and arthropods, nor radiate 

 as in the echinoderms, nor plant-like as with the sponges 

 and polyps. Where the typical moUuscan body is well 

 developed it is composed of four principal parts: a head, 

 with the mouth, feelers, eyes, and other organs of special 

 sense; a trunk containing the internal organs; a foot which is 

 a muscular mass not at all foot- or leg-like in shape, but which 

 is the organ of locomotion by means of which the mollusc 

 crawls; and a mantle which is a fold of the skin enclosing 

 most of the body and which produces the shell. Such a 

 typical moUuscan body is possessed by most of the snails. 

 But in most of the other molluscs one or more of these 

 four body-regions are so fused with some other region 

 as to be indistinguishable. In the mussels and clams the 

 head is not at all set off from the rest of the body, the cuttle- 

 fishes and octopi have no foot, the slugs have no shell. In 

 the case of some of the molluscs without external shell 

 there are inside the body the rudiments or vestiges of a 

 shell. 



With regard to the internal organs we note the constant 

 presence of three pairs of ganglia, viz., the brain, lying 

 above the pharynx, which sends nerves to the feelers, eyes, 

 and auditory organs; the pedal ganglion^ which sends 

 nerves to the foot, and the visceral ganglion, which sends 

 nerves to the viscera. This is a condition of the nervous 

 system characteristic of all molluscs. The heart is a well- 

 developed pulsating sac in the upper part of the body, com- 

 posed of either two or three chambers, and there is a well- 

 defined closed system of arteries and veins, specially com- 

 plete in the cuttlefishes and octopi. This highly developed 

 condition of the circulatory system also distinguishes the 

 molluscs from the other invertebrates. 



The shell is composed of carbonate of lime and may be 

 in two pieces, bivalved as in the oyster and clams, or in one 



