l8o ZOOLOGY 



large chambers between the mesenteries in such forms as the 

 sea-anemone thus become limited to small radial canals. 



Frequently both the tubular and the bell types are found in 

 the life history of the individuals of a single species. The 

 tubular colonial polyp produces, by asexual processes such as 

 budding or fission, the bell or medusoid forms which are sexual. 

 These may remain attached or become free swimming. They 

 produce ova or spermatozoa, or both, and from the sexual 

 union of these elements the non-sexual tubular polyp is again 

 produced. This regular alternation of sexual and sexless in- 

 dividuals is known as alternation oj generation. In some forms, 

 however, the polyp has no corresponding bell (as in hydra; 

 corals; sea-anemone), and for some bells (as in some large pelagic 

 medusae) there is no corresponding polyp stage. 



224. The nutritive processes in the Ccelenterata are marked 

 by relative simplicity. Food, consisting mainly of small organ- 

 isms and organic debris, is taken into the mouth often with 

 the assistance of tentacles. The tentacles are frequently 

 armed with numerous special cells in which are developed 

 capsules containing long stinging threads, with poisonous tips. 

 When these threads are discharged they may penetrate and 

 paralyze small organisms. They are very irritating even to 

 the human skin. In some types of nettling capsules the thread 

 forms a cork-screw coil that may take hold of the hairs or other 

 projections of the prey. They serve as organs both of defense 

 and food capture (Fig. 83). 



Digestion and circulation both take place in a general 

 cavity (gastro-vascular) lined ztith entoderm. In other words the 

 circulatory function in this group is not differentiated from 

 the digestive. In the colonial forms the gastro- vascular cavity 

 of the various polyps in the colony may be directly continuous 

 (Fig. 87). Thus a kind of cooperative digestion occurs. In 

 the medusa, the corals, and forms like anemone, the cavity is 

 much more complicated than in the tubular hydroids, on account 

 of the mesenteries. The entoderm seems to take up food from 

 the gastro-vascular cavity, in part at least, by means of the 

 amoeboid action of some of the entodermic cells a? well as by 



