A PLANARIAN WORM 77 
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 aleo fuse at their hinder ends, and the median duct thus 
