A STARFISH 117 



the base of the ray toward its tip. The actual size of these organs 

 depends entirely upon the sexual condition of the animal. In 

 young or immature animals they may be no more than half an 

 inch long or less, while in reproducing animals they may extend 

 almost to the tip of the ray. The testis of the male and the ovary 

 of the female animal do not differ from each other in general ap- 

 pearance. In the mature female, however, the ovaries have a 

 light-yellow color, and in the mature male the testes are white 

 and are less voluminous than the ovaries. 



Exercise 6. Make a semidiagrammatic drawing of the animal showing 

 the details of the digestive and reproductive systems ; label all. 



Remove the stomach and the reproductive organs from the 

 body, taking care not to injure the sinuous stone canal which is 

 at one side of the former. 



The Ambulacral System. This is the most characteristic system 

 of organs in the Echinodermata. In the starfish it consists of the 

 following organs : a circular canal, called the ring canal, surround- 

 ing the mouth ; connected with this canal are nine minute, lobated 

 sacs called the racemose or Tiedemann's vesicles, two being lo- 

 cated in each interray except the one in which is the stone canal, 

 where but one is present ; five radial canals, which pass from the 

 ring canal along the median line of the ambulacral grooves to the 

 tips of the arms ; the ambulacral feet, which are connected with 

 the radial canals by short branch canals, and also project through 

 the ambulacral pores into the body cavity, where they expand to 

 form small sacs called ampullae ; a sinuous canal, called the stone 

 canal, which connects the ring canal with the madreporite ; the 

 madreporite, a porous plate by means of which the entire system 

 is placed in communication with the outside sea water. 



In studying this system find first the madreporite and the stone 

 canal, and trace the latter from the madreporite to the ring canal. 

 Remove the spines which project over the peristome and find the 

 ring canal. It is a delicate tube, of about the diameter of a needle, 

 which surrounds the mouth, running around the base of the arms 

 at the point where the peristome joins them ; it is thus, like the 

 radial canals, outside the body cavity. Remove some of the am- 



