FLAT WORM (pLANARIA) 331 



Metabolism 



The food is principally animal tissue with some plant matter, and 

 ingestion takes place through the proboscis. The food may be par- 

 tially digested by a fluid produced in the pharynx. The principal 

 process of digestion occurs in the cavity of the enteron. Here the 

 process is similar to that of Porifera and Coelenterata, being both in- 

 tercellular and intracellular ; that is, part of the food in the intestinal 

 cavity is digested by secretions from cells in their walls, while other 

 food particles are engulfed by pseudopodia extended from cells 

 lining the cavity and are digested in food vacuoles inside the cells. 

 Absorption and assimilation take place through the plasma mem- 

 branes of adjacent cells. Since the diverticula of this system pene- 

 trate all parts of the body, and the diffusion of materials supplies all 

 other cells, no circulatory system is necessary to transport nutriment. 

 There is no anus, so all indigestible material is egested by way of 

 the mouth. Respiration is accomplished through the general surface 

 epithelium, and oxygen is distributed by diffusion through the proto- 

 plasm and fluid-filled spaces of the parenchyma. Catabolism or dis- 

 similation takes place in the cells by union of the oxygen with the 

 organic components of the protoplasm. Excretion or elimination of 

 nitrogenous waste liquids is cared for by the flame cells and system 

 of tubules. The flame cells absorb these wastes from the surrounding 

 tissues and force the fluid into the tubules by. the action of the cilia. 



Reproduction and Life History 



Sexually the individuals are hermaphroditic. The spermatozoa or 

 male germ cells mature in the testes, then pass through the vasa 

 efferentia and vasa deferentia, to the seminal vesicles where they 

 are stored in advance of copulation. Here they become organized 

 into pockets known as spermatophores. The ova mature in the 

 ovaries, pass down the oviducts where yolk cells or nurse cells are 

 added by the yolk glands, through the vagina to the genital atrium, 

 and probably from here to the uterus or seminal receptacle where 

 they are thought to be stored. Cross- fertilization or self-fertilization 

 may occur, due to the fact that both the cirrus and the vagina open 

 by way of the genital atrium. Planarians have been observed to 

 copulate with an apparent exchange of spermatozoa in the form of 

 spermatophores. In copulation the cirrus or penis is protruded 

 through the genital pore and enters the genital pore and on into the 



