The Worms and some of their Posterity 



sought out, would gradually shift to a position beside them. 

 These adaptions, it is obvious, have produced a complete change 

 in the architecture of the animal. Our sea-anemone, or Medusa, 

 or Ctenophore is radially symmetrical. That is to say, its parts 

 are arranged like the spokes of a wheel, and it may be divided 

 into two equal halves by each of 

 several planes passing through the 

 main axis. It has an upper and a 

 lower surface, but no head and tail 

 ends. The lowest of the worms 

 now can be divided into two halves 

 only in one direction, that which 

 separates the right and left sides. 

 They are, in scientific language, 

 bilaterally symmetrical. The change 

 to this type of architecture was 

 a very important step of Evolu- 

 tion, particularly in relation to 

 locomotion. Bilateral symmetry 

 was destined to remain a constant 

 feature of three of the four great 

 groups that evolved from the 

 worms. The star-fishes reverted 

 to the earlier condition. 



The next class of worms with 

 which we have to deal is that of 

 the Rotifera. In their general 

 structure, and in their excretorv 

 and sensory-nervous systems, the 

 Rotifers do not differ essentially 

 from the Turbellarians. They do differ, however, in that the 

 digestive cavity has a second opening to the exterior, at the 

 end opposite to the mouth. The advantage of this arrange- 

 ment, which was retained in the subsequent stages of Evolution, 

 is obvious, for it renders possible a much more regular and 



thorough digestive process. Instead of the food passing in, and 



6i 



Fig. 41. — A primitive flat-worm^ 



Aphanostomum ( x 50). 



a, Mouth ; g, sense organ ; /, internal 

 digestive tissue ; s, male, and <?, 

 female, reproductive glands ; with 

 ;// and_/^ external openings. 



