246 COELENTERATA 



The radial symmetry has been acquired in the two divisions 

 along different lines of descent, and has no further significance 

 than the adaptation of different animals to somewhat similar 

 conditions of life. It is not only in the animals formerly 

 classed by Cuvier as Eadiata, but in sedentary worms, Polyzoa, 

 Brachiopoda, and even Cephalopoda among the MoUusca, that we 

 find a radial arrangement of some of the organs. It is interest- 

 ing in this connexion to note that the word " polyp/' so frequently 

 applied to the individual Coelenterate animal or zooid, was 

 originally introduced on a fancied resemblance of a Hydra to a 

 small Cuttle-fish (i'V. Poulpe, Zat. Polypus). 



The body of the Coelenterate, then, consists of a body- 

 wall enclosing a single cavity (" coelenteron "). The body-wall 

 consists of an inner and an outer layer of cells, originally called 

 by Allman the " endoderm " and " ectoderm " respectively. 

 Between the two layers there is a substance chemically allied 

 to mucin and usually of a jelly-like consistency, for which the 

 convenient term " mesogloea," introduced by G. C. Bourne, is 

 used (Fig. 125). 



The mesogloea may be very thin and inconspicuous, as it is in 

 Hydra and many other sedentary forms, or it may become very 

 thick, as in the jelly-fishes and some of the sedentary Alcyonaria. 

 When it is very thick it is penetrated by wandering isolated 

 cells from the ectoderm or endoderm, by strings of cells or by 

 cell-lined canals ; but even when it is cellular it must not be 

 confounded with the third germinal layer or mesoblast which 

 characterises the higher groups of animals, from which it diifers 

 essentially in origin and other characters. The Coelenterata are 

 two-layered animals (Diploblastica), in contrast to the Metazoa 

 with three layers of cells (Triploblastica). The growth of the 

 mesogloea in many Coelenterata leads to modifications of the 

 shape of the coelenteric cavity in various directions. In the 

 Anthozoa, for example, the growth of vertical bands of mesogloea 

 covered by endoderm divides the peripheral parts of the cavity 

 into a series of intermesenterial compartments in open com- 

 munication with the axial part of the cavity ; and in the jelly- 

 fishes the growth of the mesogloea reduces the cavity of the 

 outer regions of the disc to a series of vessel-like canals. 



Another character, of great importance, possessed by all 

 Coelenterata is the " nematocyst " or "thread-cell" (Fig. 124). 



