170 TEXTBOOK OF ZOOLOGY 



notably quite elaborate, and it is found generally that the flatworms 

 have a highly specialized reproductive system. 



The planarian worms and the representatives of this phylum pos- 

 sess no skeletal system, no respiratory system (breathe through 

 the skin); no coelom or body cavity; and no circulatory system; 

 this function, however, is performed by the branched enteron. It is 

 significant that the reproductive system upon which the continuance 

 of the race depends is highly specialized, this succeeded by the diges- 

 tive system responsible for nourishment of the individual, and this 

 followed by the nervous system which relates the organism to its 

 surroundings. 



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 necessaiy 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 



