40 Laboratory Guide in Zoology 



of locomotion, would you say that it possesses anterior 

 and posterior parts ? 



XIX. — Tlie blood system you will probably be unable to 

 make out. It has the same general arrangement as 

 the nervous and water systems, that is, a circular ring 

 around the peristome, which sends out five branches of 



t vessels, one to each ray. 



XX. — Dissect each of the remaining rays, varying the 

 operation to suit the necessity of clearing up any 

 points not at first understood. Make drawings. 



THE SEA-URCHIN 



The Sea-urchin lives along the seacoast, and may often 

 be found abundantly in pools among the rocks when the 

 tide is out. Two species are common along the Atlantic 

 coast. 



I. — Notice the general appearance of the animal — the 



spheroidal shape of the body, and the spines with 

 which it is covered. See that one side is more flat- 

 tened than the other ; this is the oral surface, while 

 the other is the aboral surface. 



II. — Examine the oral surface carefully. See the central 

 mouth opening, with its five projecting calcareous teeth, 

 and the more or less membranous area surrounding it; 

 this area is the peristome. See whether that part of 

 the peristome next to the mouth opening is thickened. 

 Find also the small calcareous ossicles which give 

 strength to the membranous peristome. Make a draw- 

 ing of the peristome and mouth. 



III. — Find along the outer margin of the peristome a great 

 many of the movable ambulacra or tube feet, each end- 

 ing in a sucker-like disk. Each of the ambulacra con- 

 sists of a cylindrical basal portion, which in the living 



