142 PHYLUM CCELENTERA. 



The sea-anemones and their numerous allies may be 

 regarded as bearing a relation to the jelly-fish, somewhat 

 similar to that which the hydroid polyps bear to the 

 swimming-bells (Fig. 65). They are, however, much more 

 complicated in structure than the hydroids. Solitary forms 

 are much commoner than in the hydroids, but the colonial 

 type is nevertheless very frequent. The colonies may be 

 supported by an organic framework only, but very commonly 

 there is a tendency to accumulate lime in the tissues, which 

 results in the formation of " corals." It should be noted, 

 however, that various quite distinct polypoid types may 

 form " corals." Thus, while the most important reef-building 

 corals are included in the Anthozoa, the Millepore-corals 

 We hydroids. 



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 amongst molluscs ; but it has been 

 shown that these animals obtain their nematocysts from the Coelentera 

 on which they feed. Each cnidoblast contains a capsule or nemato- 

 . cyst, 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 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 



