136 INVERTEBRATE ZOOLOGY 



to the outside through minute pores in the aboral walls of the 

 interrays. Two glands will be found in each ray extending 

 from 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 appearance. In the mature female, however, 

 the ovaries have a light-yellow color, while in the mature male 

 the testes are white and are less voluminous than the ovaries. 



Exercise 7. 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, 

 surrounding the mouth ; connected with this canal are nine 

 minute lobated sacs called the racemose or Tiedemann's vesicles, 

 two being located 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 madreporic plate ; the madreporic plate, a porous 

 plate by means of which the entire system is placed in commu- 

 nication with the outside sea water. 



