Flatworms and Roundworms 373 



The female organs include a pair of spherical ovaries near the anterior 

 end. Each is connected by means of an oviduct with the genital cham- 

 ber. A series of yolk glands empty yolk (food) for the eggs into the 

 oviduct. After copulation the sperm leave the seminal receptacle, travel 

 up the oviducts toward the ovaries, and fertilize the eggs as they leave 

 the ovary. As the fertilized eggs pass down the oviducts, they are sur- 

 rounded by yolk cells from the yolk glands. In the genital chamber 

 clumps of eggs and yolk cells are surrounded by a shell-like egg capsule 

 (cocoon). Each cocoon may contain as many as ten eggs and hundreds 

 of yolk cells for nourishment. The cocoons are passed to the outside 

 through a genital pore. The eggs develop in two to three weeks into 

 miniature worms without a reproductive system. The reproductive or- 

 gans of adults degenerate after each breeding and are later regenerated. 

 This might explain the absence of reproductive systems in certain adults. 



Planaria also reproduces asexually by transverse fission. Ordinarily 

 Planaria constricts just behind the pharynx, and after several hours the 

 two parts separate. The anterior part regenerates the missing posterior 

 part, while the posterior part regenerates an anterior part, with all miss- 

 ing structures eventually replaced. Because of axiate organization, 

 Planaria shows a remarkable ability to replace missing parts. This axiate 

 gradient of metabolic activity can be demonstrated by the relative 

 amounts of oxygen used, and the amounts of carbon dioxide given off", 

 by the different levels from one end of the animal to the other. Planaria 

 may be carefully cut into pieces to illustrate this remarkable ability to 

 regenerate. If properly conducted, this makes one of the most interest- 

 ing exercises in the laboratory. 



LIVER FLUKE 



The liver fluke, Fasciola (Distomum) hepatica, is a parasitic trema- 

 tode (tre'matode) (Gr. trema, pore; eidos, resemblance) which in its 

 adult stages may live in the liver of sheep, cows, pigs, and occasionally 

 man. The immature stages are found in the bodies of a specific kind 

 of water snail, with a few special stages in water, or on vegetation. The 

 adults are flat and leaf like. The body is triploblastic (Figs. 180, 181, 

 374). 



Integument. — The ectoderm is a thick, heavy, elastic cuticle which 

 protects the adult from the host which it parasitizes. The entoderm 

 lines the alimentary tract. The mesoderm is represented by muscles, 

 excretory organs, reproductive organs, and parenchyma. The paren- 



