Flat worms and Threadworms 



289 



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Fig. 178. Regeneration of Planaria maculata. 

 A, normal worm. B, B 1 , regeneration of 

 anterior half. C, C 1 , regeneration of posterior 

 half. D, cross-piece of worm. D 1 , D 2 , D 3 , D*, 

 regeneration of same. E, old head. E 1 , E 2 , E 3 , 

 regeneration of same. F, F 1 , regeneration of 

 new head on posterior end of old head. (From 

 Hegner after Morgan.) 



The ovaries are two in number. From these the two long oviducts 



(which possess many yolk-glands) connect with the vagina. The vagina 



opens into the genital cloaca. The uterus also connects with the cloaca. 



After the eggs ripen, they pass from the ovary through the oviducts 



(where they collect yolk from the 

 yolk-glands) and finally reach the 

 uterus. Fertilization occurs in the 

 uterus. Cocoons are formed, each 

 containing from four to twenty eggs 

 and several hundred yolk-cells. 



As already stated, Planaria may 

 also reproduce by fission. This 

 means in this instance that when 

 the hindermost portion of the ani- 

 mal is grown, it breaks off from the 

 fore part to produce a new animal. 

 Regeneration. 



From the laboratory point of 

 view, Planaria is probably the most 

 available animal one can find to 

 show regeneration experiments. 

 This is especially true because the parts to be regenerated grow very 

 rapidly, each day marking a definite growth region. 



Almost any part of the animal will re-grow, but there are portions 

 quite specialized in what is re-grown. If, for example, the head is cut 

 off directly behind the eyes, the more anterior part will regenerate a 

 new head but no body, thus making a two-headed animal. Such speciali- 

 zation is called polarity. (Fig. 178.) 



Two types of eggs are laid. In the summer the eggs are thin- 

 shelled and develop quickly, while in the autumn the "winter eggs" are 

 laid. These are thick-shelled and lie dormant until spring before 

 hatching. 



TREMATODA 



All the trematodes are parasitic. Some are monogenetic; that is, 

 the adults lay eggs which hatch into forms like their parents, all living 

 on the outside of their host. These are said to have a simple life-history. 

 This type of animal is usually found on cold-blooded vertebrates, such 

 as frogs, fishes, etc. 



The endoparasitic trematodes (those which live in the internal or- 

 gans of a host, whether that be in the liver, lungs, intestines, bladder 

 or other similar internal structure), are mostly digenetic. This means 

 that the parasite must pass through more hosts than one to complete its 

 life cycle. 



The liver fluke, Fasciola hepatica (Fig. 179) is the form usually 

 studied in the laboratory. 



