138 PHYLUM CCELENTERA. 



Finally, as the corals are predominantly passive, so there 

 is a climax of activity in the Ctenophores, which move by 

 cilia united into combs, and often shine with that "phos- 

 phorescence " which is an expression of the intensity of life 

 in many active animals. 



As to diet, many of the larger forms, e.g. sea-anemones 

 and jelly-fish, are able to engulf booty of considerable size ; 

 the active Ctenophores are carnivorous, attaching them- 

 selves by adhesive cells to one another, or to other small 

 animals ; but most Coelentera feed on small organisms, in 

 seizing and killing which the tentacles and stinging cells 

 are actively used. 



Stinging cells or cnidoblasts are so characteristic of Coelentera that 

 they deserve particular notice. They occur in all Coelentera except 

 the Ctenophores, and even there they have been detected in Euchlora 

 rubra. They also occur in some Turbellarian worms, and in the 

 papillae of /Eolid nudibranchs among molluscs. Each cnidoblast con- 

 tains a capsule or nematocyst, which encloses a coiled lasso lying in 

 an irritant gelatinous substance. The nematocyst fills most of the cell, 

 but there is a nucleus, etc., besides. At the distal end there may be a 

 trigger-like cnidocil or a fringe of bristles, etc. At the proximal end 

 there may be fixing processes. In some Anthozoa the coiled lasso is 

 simply ruptured out, but in most cases it is evaginated. The basal part 

 of the lasso is often stronger than the rest, and may bear stilets ; spirally 

 arranged roughnesses and bristles are also frequent on the thread itself. 

 The explosion of the cnidoblast is believed by most authorities to be due 

 to an entrance of water, which causes the gelatinous substance to swell 

 up. According to others, the cnidoblast contracts as a whole. The 

 action of the threads is both mechanical and chemical. They are fixed, 

 e.g. by help of the stilets, into the victim, and the irritant substance 

 poisons the wound, causing paralysis or death in small animals. 



TYPES OF CCELENTERA. 



First Type. HYDRA, a simple representative of the 

 Class HYDROMEDUS^.. 



General life. - The genus Hydra is represented by 

 several species, e.g. the green Hydra viridis and the 

 brownish Hydra fusca, both widely distributed in fresh 

 water. They are among the simplest of Coelentera, for the 

 body is but a two-layered tube, with a crown of (6-10) 

 hollow tentacles around the mouth, and with no organs 



