PHYLUM IV. -MOLLUSCA. 



Oysters, clams, snails, and cuttlefish may be taken as 

 examples of the ten thousand different species which are 

 known as molluscs. The name comes from the Latin 

 mollis, soft, and alludes to the fact that, aside from the 

 shell, the body has no conspicuous hard parts. This, how- 

 ever, is a point of no real importance in classifying animals. 



Molluscs vary greatly in appearance; but if we carefully 

 compare the points which all possess in common, we can 

 construct an ideal mollusc, from which anv form mav be 



/ / 



derived by additions here and modifications there. Such 

 a typical mollusc is described below (fig. 34). 



FIG. 34. Transverse and longitudinal sections of a schematic Mollusc, a, auri- 

 cle; c, cerebral ganglia; d, digestive tract; /.foot; g, gill; h, heart; i, intes- 

 tine; I, liver m, mouth; n, nervous system; p, pedal ganglia; pc, pericar- 

 dium; s, stomach; v, vent (in left figure, ventricle). 



The body is saccular, and bilaterally symmetrical. 

 There is, above, a conical visceral mass; below, a muscular 

 foot; while from either side a fold of the body-wall extends 



outwards and downwards as a mantle. Between the 



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