i3o 



CCELENTERA. 



Slinging cells or cnidoblasls are so characteristic of Ccelentera that 

 they deserve particular notice. They occur in all Ccelentera 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 yEolid 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. 



Tvpes of Ccelentera. 

 First Type. — Hydra, illustrative of the Class Hydrozoa. 



General life. 

 several species, 



The genus Hydra is represented by 

 f. the green Hydra viridis and the 

 brownish Hydra fusca, both widely 

 distributed in fresh water. They are 

 among the simplest of Ccelentera, for 

 the body is but a two-layered tube, 

 with a crown of (6-10) hollow tentacles 

 around the mouth, and with no organs 

 except those concerned in reproduc- 

 tion. The body is usually fixed by its 

 base to some aquatic plant, often to 

 the underside of a duckweed. It may 

 measure £-| in. in length, but it is as 

 thin as a needle, and contracts into a 

 minute knob. 



The animal sways its body and 

 tentacles in the water, and it can also 

 loosen its base, lift itself by its tentacles, 

 stand on its head, or creep by looping movements. Ac- 

 cording to some observers, its movements may be helped 

 by fine pointed pseudopodia protruded from the ectoderm 



Fig. 56. — Hydra hang 

 ing from water-weed 

 — After Greene. 

 ov. y Ovary; t, testes. 



