212 



ZOOLOGY 



derms (such as the sea urchin) is secondary or derived, 

 a response to the needs of sedentary life. These 



Two modes 

 of repro- 

 duction 



B 



Drawing by R. Weber 



FIG. 47. Diagram illustrating the radial symmetry of a starfish (Echinodermata) , 

 A ; and a medusa (Cdlenterata), B. 



facts are determined from a study of the early stages, 

 but it is also to be noted that the radial segmentation of a 

 jellyfish is fundamentally different from that of an echi- 

 noderm. In the echinoderms, as A. H. Clark pointed 

 out, the divisions are lines of weakness ; hence the typi- 

 cally five-rayed condition, which provides that no such 

 line will go straight across the body. In the jellyfish 

 the divisions are marked by lines of greater strength, and 

 hence when continued across the body give added rigid- 

 ity. We therefore find quadripartite ccelenterates. 



4. The primitive character of the ccelenterates is 

 shown also by their modes of reproduction. They pos- 

 sess sex, but also reproduce by budding. Individuals 

 are produced as lateral buds, which live for a time as 

 parasites attached to the parent, and finally become de- 

 tached and independent. This is a natural process, but 

 a fresh-water Hydra may be cut up into a number of 

 pieces, and each one will grow into a perfect individual. 

 In certain groups the asexual mode of reproduction is 

 lost, and there are separate sexes as in higher animals. 

 In the fresh-water Hydra the male and female genera- 

 tive cells may be produced by the same individual, when 



