A PLANARIAN WORM 71 



Study an animal which is under the pressure of a large 

 cover-glass, and make out as many of the following organs as 

 possible, using often reflected instead of direct light : 



The digestive system. The digestive canal is usually easily 

 seen. The mouth is a circular opening near the center of the 

 ventral surface ; it leads into the pharynx, a cylindrical organ 

 with thick, muscular walls, which can be thrust out of the 

 mouth as a proboscis. At the base of the pharynx the intestine 

 divides into three trunks, one of which passes forward, and the 

 other two backward to the extremities of the animal's body. 

 Each of these trunks gives off lateral branches which are them- 

 selves often branched. There is no anus. 



Exercise 2. Draw an outline of the animal and place in it the 

 digestive system in detail. 



The reproductive system. Planarians are hermaphroditic; the 

 sexual organs are complicated in structure and arrangement 

 and difficult to observe in a live specimen. Near the lateral 

 edges of the body will be seen, among the ends of the lateral 

 intestinal branches, two sets of lobed organs. Of these the 

 larger are the yolk glands, which connect with the oviducts ; the 

 smaller and less apparent ones are the rounded testes. Just 

 back of the mouth is the uterus, which is often to be recognized 

 by the spherical eggs it may contain ; it passes back to a sac 

 called the genital cloaca. The ovaries are a pair of spherical 

 bodies in the anterior part of the body, and from them a pair of 

 oviducts extends to the hinder part of the body, receiving the 

 lateral yolk glands on their course. Leading from the testes are 

 the vasa efferentia, very delicate tubes, which pass to the con- 

 spicuous vasa deferentia. There is a pair of the latter organs, one 

 on each side of the mouth and pharynx, and they extend to 

 the hinder part of the animal, where they unite to form the 

 muscular cirrus, which opens into the genital cloaca. The two 

 oviducts also fuse at their hinder ends, and the median duct thus 



