166 



THE RISE OF ANIMAL LIFE 



Fig. 9-7. Regeneration in planaria. 



duction. The worm constricts itself in two 

 and the parts which are missing after the 

 fission has taken place are then regener- 

 ated. Planaria is an excellent animal for 

 regeneration experiments. For example, if 

 the head of planaria is split and if the parts 

 are kept separated for a short period of 

 time, a double-headed monster is formed 

 (Fig. 9-7). Should the animal be cut trans- 

 versely into two separate pieces, two little 

 planaria will result. An animal can be cut 

 into as many as six pieces and each will 

 give rise to a miniature worm one-sixth the 

 size of the original. The significance of re- 

 generation was pointed out in an earlier 

 section. 



OTHER TURBELLARIA 



Although members of the class Turbel- 

 laria are, for the most part, free-living, some 

 live on the exterior of other animals and 

 others are true parasites living in the in- 

 testines of mollusks and various echino- 

 derms, such as the sea cucumber. Economi- 

 cally, they are of little importance. 



The small primitive marine forms of 

 the order Acoela, have a mouth but no di- 

 gestive system, the food being digested by 

 the endodermal cells. Convoluta, an animal 

 that lives on sandy ocean shores, is a good 

 example of this order. As it matures, algae 

 enter and inhabit its parenchymous tissue, 

 giving it a green color. BdeUtira, referred 

 to earlier (p. 95), is also a member of this 

 group. 



Members of the order Rhabdocoelida are 

 turbellarians with a straight tubular gut. In 

 certain forms the gut has lateral pouches, 

 but not as highly branched as that of pla- 



naria. These turbellarians are commonly 

 found in fresh water, but because of their 

 small size are not as easily studied as 

 planaria. The largest of these forms is 

 Mesostoma, which, like members of the 

 order Tricladida, has its mouth on the 

 ventral surface. The smaller rhabdocoeles 

 have their mouths in the anterior region, 

 ventral to the brain. Some of these animals 

 have such remarkable powers of reproduc- 

 tion by fission that individuals form long 

 chains which remain t02;ether for some time 

 before separating. This was mentioned ear- 

 lier as a possible explanation of the origin 

 of segmentation. 



Members of the order Polycladida are 

 marine forms, usually small, although some 

 attain a leno;th of 6 inches. This order in- 

 eludes a few rare species found only in iso- 

 lated places, such as the Gulf of Naples. 

 The digestive system is well branched and 

 the body is unusually flattened. In some of 

 the simpler forms, the mouth is centrally 

 located and the pharynx, which is funnel- 

 like in structure, can be extended from the 

 mouth to enclose food. One difference be- 

 tween this order and the others is that 

 development is not direct, but must pass 

 through a rounded, ciliated larval form 

 which possesses projecting arms. As the ani- 

 mal grows, it loses the cilia and the arms, 

 develops a crawling movement, and be- 

 comes considerably lengthened and flat- 

 tened. 



CLASS TREMATODA 



The trematodes, commonly called flukes, 

 are characteristically flat like all platyhel- 

 minthes, but their gut is reduced in com- 

 plexity. Because of their parasitic life they 



