286 ELEMENTS OF COMPARATIVE ZOOLOGY. 



radial canal running below the arm in the median line. 

 These radial canals unite to form a ring-canal around the 

 mouth, and this in turn communicates with a stone-canal 

 which leads to the aboral surface, and thence to the exterior 

 through pores in a specialized plate, the madeporite. This 

 whole system is known as the water-vascular system. By 

 means of ampullar muscles the ambulacra can be ex- 

 tended, while ambulacral muscles serve for their retraction. 

 At the end of each ambulacra is a sucking-disc. 



The nervous system consists, chiefly, of a nerve-ring 

 around the mouth and a radial nerve in each ray, the 

 whole paralleling the water-vascular system. Eye-spots, 

 one at the end of each ray, are the only specialized sense- 

 organs present. 



The circulatory organs consist of a so-called heart beside 

 the stone-canal, from which vessels run in various directions, 

 the chief portion running between nervous and water- 

 vascular tracts. The only respiratory organs are the thin- 

 walled branchiae, which are outpushings of the body-cavity 

 upon the dorsal surface. 



The reproductive organs occur at the bases of the arms, 

 one organ on either side of each ray, the ducts emptying 

 in the angle between the arms. From the eggs there hatch 

 out larvae which are free-swimming and bilateral, and 

 which show not the slightest trace of the radial shape of 

 the parent. 



The starfishes are all marine. They feed largely on 

 clams, oysters, and other molluscs, and are regarded as one 

 of the greatest pests on oyster-beds. The way in which the 

 starfish feeds is interesting. It has no hard parts to break 

 the shell, while the mouth is too small to admit of swallow- 

 ing the oyster. So it everts its stomach through the 

 mouth and wraps it around the shell it wishes to devour. 



