172 ZOOLOGY. 



radiating passages run through the substance of the bell to 

 the margin where they may communicate with a circular canal 

 which passes around the bell near the bases of the tentacles. 

 This whole internal cavity is lined with entoderm, and there- 

 fore no portion of it represents a ccelom, but is merely a much- 

 modified digestive tract (Fig. 80, 6). 



The bell is comparable to an inverted polyp in which the 

 main axis has become much shortened, accompanied by a 

 thickening of the body in the direction of the other axes. 1 

 The gastro-vascular cavity is further modified by the increase 

 of the mesenchyma of the aboral disc and by a union of the 

 oral and aboral walls of the cavity in certain regions. The 

 large chambers between the mesenteries in such forms as the 

 sea-anemone thus become limited to small radial canals. Fre- 

 quently both of these 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 sexuaU 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 individuals is known 

 as alternation of 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 me- 

 dusae) there is no corresponding polyp stage. 



219. The nutritive processes in the Ccelenterata are 

 marked by relative simplicity. Food, consisting mainly of 

 small organisms and organic debris, is taken into the mouth 

 often with the assistance of tentacles. The tentacles are fre- 

 quently armed with numerous special cells in which are de- 

 veloped capsules containing long stinging threads, with barbs 

 or poisonous tips. These may be everted and possibly their 

 action brings about a partial paralysis of the prey. They 



'See Text-Book of Zoology, Parker and Haswell, Vol. I, p. 127, Fig. 

 89- 



