GENERAL CHARACTERS OF CCELENTERATA. 125 



speak of many of the interesting problems connected with 

 corals. How do they get their carbonate of lime ? Is that 

 salt peculiarly abundant about coral reefs, or is there, as 

 Irvine and Murray suggest, an interaction between the waste 

 products of the polypes and the sulphate of lime abundant 

 in sea-water? On what do they feed? Do their bright 

 pigments, as Hickson suggests, enable them to utilise 

 carbonic acid after the manner of plants ? Then there is 

 the struggle for standing-room among the coral polypes, and 

 the struggle for existence among the many brightly-coloured 

 animals which browse and hide on the reef. 



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 intensity of life in 

 many active animals. There are hints of the way in which 

 the Ctenophores may have arisen from a medusoid type. 



General Characters of Cmlenterata. — (i) In Ccelenterata 

 the symmetry of the gastrula is usually preserved ; the long 

 axis of the hydroid, sea-anemone, or jellyfish is the vertical 

 axis of the gastrula, and the body is almost always radially 

 symmetrical round this. (2) The simplest forms do not in 

 fltructure rise much above the gastrula level ; and the fre- 

 quently jelly-like supporting substance or " mesogloea," very 

 generally found between the outer and inner layers of cells, 

 cannot be regarded as a true mesoderm. (3) There is no 

 body-cavity distinct from the primitive alimentary cavity of 

 the gastrula and outgrowths from it. In these three respects 

 the Ccelenterata differ from almost all the other Metazoa, to 

 which the name Ccelomata is sometimes applied as a con- 

 trasted title. (4) There is an almost constant presence of 

 offensive and defensive stinging-cells, whence the vivid 

 German name for the class — nettle-animals. (5) Vegetative 

 multiplication by budding is very common, especially in the 

 passive hydroids, the plant-like appearance of which gave 

 origin to the old name of zoophytes. (6) The life-history 

 frequently illustrates, in its rhythm of activity and passivity, 

 in its combination of free-swimming sexual and sessile asexual 

 forms, what is known as alternation of generations. (7) In 

 different classes " corals " occur, usually in passive forms, 

 and usually with calcareous " skeletons." 



