FROM SINGLE CELLS TO MANY CELLS 



79 



(Fig. 4-10). It functions both as a digestive 

 cavity and as a space where food can be cir- 

 culated to the Hning cells; therefore, it is 

 referred to as a gastrovascular cavity as 

 well. Such a cavity has but one opening 

 through which food must pass upon enter- 

 ing and undigested food upon leaving the 

 body. The flatworms also possess such a 

 cavity. 



As animals increase in complexity, the 

 simple sac-like coelenteron of the lower 

 forms is inadequate and becomes modified 

 into an enteron with an additional opening, 

 the anus, at the opposite end from the 

 mouth. Thus food can follow a one-way 

 path through the body, a more efficient 

 arrangement, certainly, than the crude 

 coelenteron of Hydra. In the earthworm 

 and higher forms another cavity, the coe- 

 lom or body cavity, appears, which lies be- 

 tween the digestive tract ( enteron ) and the 

 body wall. It is a convenient space into 

 which wastes, sex cells, and some foods can 

 be dumped, later to be eliminated or uti- 

 lized. It also provides space for the internal 

 organs which become much more compli- 

 cated as animals grow more complex. The 

 entire body cavity is lined with a thin sheet 

 of tissue called peritoneum. Once the coe- 

 lom had arisen in those early invertebrates 

 somewhat like the earthworms, it appar- 

 ently proved highly satisfactory in further- 

 ing the development of animals, for it was 

 retained through all higher groups of ani- 

 mals. 



Segmentation. Another feature that was 

 introduced early among animals and re- 

 tained throughout subsequent groups was 

 the clinging together of individuals, form- 

 ing long chains which resulted in seg- 

 mented or metameric animals. Certain low 

 worms divide by transverse fission but fail 

 to separate until a large number of fissions 

 have taken place, thus producing a long 

 contiguous series of worms clinging to- 

 gether head to tail ( Fig. 4-11). It is thought 

 that some of these failed to separate at all, 

 thereby producing a long wonu consisting 



• « 



Fig. 4-11. A possible explanation of the origin of 

 segmentation. 



