Chap. III.] 



CTENOPHORA. 



45 



calcareous salts often gives rise to large masses of 

 " stony " coral, of which the brain-coral (Mseandrina) 

 is a good example ; in other cases (e.g. Fungia) ths 

 septa are alone calcined. 



There still remains a division of the Ccelenterata 

 which, though it has been definitely placed by some 

 naturalists with the Hydrozoa, 

 and by others with the Antho- 

 zoa, is possibly an independent 

 group ; in these, the eight canals 

 derived from the enteron run 

 at equal distances close to the 

 surface of the body, and along 

 these there are formed bands of 

 cilia, which have, in consequence 

 of their comb-like appearance, 

 gained for these forms the name 

 Ctenopliora. The glassy globe 

 called Cydippe (Fig. 15) is found 

 on our own shores, while Venus' 

 girdle (Cestus veneris) is an ex- 

 ample of that acquired bilateral 

 symmetry to which we have already 

 referred (Fig. 16). 



Fig. 15. Cydippepileus. 



THE HIGHER METAZOA. 



In the remaining Metazoa a cavity distinct from 

 the archenteric cavity becomes developed, and the 

 mesoblast becomes the seat of those important changes, 

 by means of which nearly all the tissues of the body 

 are derived from it. In the midst of this mesoblast 

 a cavity arises by cleavage or fissure, or from the 

 archenteron there are given off out-growths which, 

 in time, become shut off from the parent space, and 

 occupy the middle of the mesoblast. The cavity 

 formed in either of these ways is spoken of as the body 

 cavity or ceelom, and the result of its appearance is 



