174 



A COURSE ON ZOOLOGY. 



has violet spines, is very abundant, and is used as food 



in some maritime localities. 



The sea-urchin has a mouth provided with calcareous 



joints shaped like 



FIG. 102. little pyramids, the 



summits converging 

 to a point. These 

 form a sort of teeth, 

 and, being put in 

 movement by the 

 action of muscles, 

 serve to tear up the 

 food. The solid 

 frame - work has 

 been called Aris- 

 totle's lantern. The 

 digestive tube is 

 well developed and 

 tortuous. 



The asterias or 

 star -fish owes its 

 name to the form 

 of its body, which is 

 divided into five or 

 ten rays. In some 

 species these rays 

 are subdivided indef- 

 initely, and form a 

 sort of tangled hair. 



UNDER SIDE OF A STAR-FISH (the rays are cut 

 open to show the organs). b, mouth ; d, ap- 

 pendages of the intestine ; e, stomach ; an, 

 cesophageal ganglia ; av, annular canal, mad- 

 reporic (perforated) plate ; t, open covering. 



Sometimes these star fish are found in such numbers that 

 they are used as manure for the fields. Like the sea- 

 urchins, they are very voracious ; the mouth is in the 

 central portion, and leads by a short oasophagus into an 



