90 INVERTEBRATE ZOOLOGY 



ticula have the form of definite vessels, called the radial canals, 

 which may be united at their distal ends by a common vessel 

 termed the circumferential canal. Further complication of the 

 digestive system is found in the Anthozoa wherein a definite 

 infolding of ectoderm projects for some distance from the mouth 

 opening into the digestive chamber as an esophagus. In this 

 same group, the cavity becomes divided by a series of partitions 

 called mesenteries or septa which provide additional surface 

 for the processes of digestion and assimilation of the food 

 material. 



Metagenesis. — Both types of individuals described above 

 occur in the course of the life cycle of the Hydrozoa and the 

 Scyphozoa. An asexual hydroid generation gives rise to a sexual 

 medusoid generation. This condition of direct alternation 

 between two generations of different type has been termed meta- 

 genesis. The occurrence of hydroid and medusoid forms in 

 the developmental cycle of a single species has not been under- 

 stood long. It is, then, not surprising that all of the earlier 

 treatises considered the hydroid and medusoid generations, 

 even of the same species, as belonging to distinct and independent 

 groups of the animal kingdom. 



Habitat. — In habits, the coelenterates are chiefly marine, 

 though Hydra, Protohydra, Cordylophora, and a few rare 

 medusae occur in fresh-water habitats. 



At the seashore, the anemones and the colonial hydroids are 

 among the most interesting animals on pilings, on seaweeds, and 

 on rocks, while the fragile bodies of the free-floating jellyfishes 

 with their play of colors and graceful pulsations are the most 

 conspicuous of the free-floating plankton organisms. Because of 

 their size and abundance, coelenterates are among the most prom- 

 inent of the organisms which render the ocean phosphorescent. 



Food Relations. — Most coelenterates feed upon microscopic 

 organisms which are captured by the tentacles and killed by the 

 nettling cells. Fishes and crustaceans of considerable size are 

 included in the food of some of the jellyfishes, however, and even 

 the minute fresh-water hydras at times feed upon newly hatched 

 fish. In turn, the soft bodies of coelenterates frequently fall 

 prey to other animals in spite of the protective nature of the 

 nettling cells. The discovery of nettling cells in some flat worms 

 and molluscs threw some doubt upon the nematocysts as dis- 

 tinctive organs of the coelenterates but it is now known that the 



