MOLLUSCA : THE MOLLUSCS 245 



Body form and structure. The molluscs are not to 

 be mistaken for any other of the lower animals ; they 

 have a structure 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 

 molluscan 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 thick 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 

 molluscan 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 

 composed of either two or three chambers, and there is a 

 well-defined closed system of arteries and veins, specially 

 complete in the cuttlefishes and octopi. This highly 

 developed condition of the circulatory system also distin- 

 guishes the molluscs from the other invertebrates. 



