The Development of o Primitive Animal 39 



the sack ( coelenteron ) is elongated and narrow with its open end up. In 

 the medusa, the sack is squat and rounded with its open end down. Some 

 species appear both as polyps and medusae during their life cycles. In 

 other species, one or the other adult form may be rudimentary and very 

 short-lived or entirely absent. 



In both forms, the mouth is surrounded by tentacles, and these con- 

 tain special stinging cells called cnidoblasts (hence the name Cnidaria). 

 Upon stimulation, they shoot out spear-like processes bearing powerful 

 toxins to paralyze their prey (small crustaceans, insect larvae). Very little 

 is known about the toxins. The few that have been studied appear to be 

 proteins. When injected into rabbits, they produce neural damage and 

 elicit the formation of antibodies, just as do certain snake venoms, tetanus 

 toxin from bacteria, and other poisonous proteins. 



Life Cycle of the Genus Obefio 



Both polypoid and medusoid stages figure prominently in the life 

 cycle of Obelia, which is summarized in Fig. 20. The polyps grow as colo- 

 nies upon rocks and shells in shallow marine waters. The colony is 

 fastened to the substratum by a mass of root-like runners that bear slender, 

 branched stems, and from these extend polyps of two kinds: the feeding 

 polyps equipped with mouth and tentacles, and the reproductive polyps, 

 which lack all feeding mechanisms and are designed for one purpose— to 

 produce medusae that bud off from the central cylinder and escape 

 through the hole in the vase-shaped outer covering. 



Fig. 20. The life cycle of Obelia (after S+orer and Usinger). 

 POLYP - MEDUSAE 



