106 MARINE ANIMALS OF MASSACHUSETTS BAY. 



away of the refuse matters ; nor do the forks cease from their 

 labor till the surface of the animal is completely clean, and free 

 from any foreign substance. Were it not for this apparatus the 

 food thus rejected would be entangled among the tentacles and 

 spines, and be stranded there till the motion of the water washed 

 it away. These curious little organs may have some other office 

 than this very laudable and useful one of scavenger, and this 

 seems the more probable because they occur over the whole surface 

 of the body, while they seem to pass the excrements only along 

 certain given lines. They are especially numerous about the 

 mouth, where they certainly cannot have this function ; we shall 

 see also that they bear an important part in the structure of the 

 Star-fish, where there are no such avenues on the upper surface, 

 for the passage of the refuse food, as occur on the Sea-urchin. 



On opening a Sea-urchin, we find that the teeth (Fig. 138), 

 which seem at first sight only like five little conical wedges 

 around the mouth (Fig. 134), are connected 

 ^1^^^^^^^ with a complicated intestine, which extends 

 spirally from the lower to the upper floor of 

 the body, festooning itself from one ambula- 

 cral zone to the next, till it reaches the sum- 

 mit, where it opens. This intestine leads into 

 the centre of the teeth, the jaws themselves, 

 which sustain the teeth, being made up of a 

 number of pieces, and moved by a complicated 

 system of muscular bands. When the intes- 

 tine is distended with food, it fills the greater part of the inner 

 cavity ; the remaining space is occupied in the breeding season 

 by the genital organs. In a section of the Sea-urchin, one may 

 also trace the tube by which the supply of water, first filtered 

 through the madreporic body, is conveyed to the ambulacra ; it 

 extends from the summit of the body to the circular tube sur- 

 rounding the mouth. 



Echinarachnius. (Echmarachnius parma Gray.) 



Beside the Toxopneustes (Fig. 131) described above, we have 

 another Sea-urchin very common along our shores. Among 



Fig. 138. Teeth of Sea-urehin, 9o-calle<l L.intem of Ariatotle. 



