280 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



" gemmules " of the fresh- water -sponge forming the 

 " amphidiscs," which strengthen its protective coat 

 during the period of quiescence. Sponges free from 

 calcareous or siliceous spicules, and with 

 only a fibrous skeleton, have, in the 

 present period, some commercial value, 

 in consequence of their well-known use 

 to man. 



From the share that they have had 

 in forming parts of the earth's crust, 

 there is no division of the animal king- 

 dom in which, from such a point of view, 

 skeletal structures are of more impor- 

 tance than in the Ccelenterata, and both 

 Hydrozoa and Anthozoa contain groups, 

 members of which form the hard struc- 

 tures which we call coral ; this con- 

 sists essentially of deposits of carbonate 

 of lime in the organic substance of the 

 body. 



The division of the Anthozoa con- 

 tain the larger number of coral-forming 

 animals, and may therefore be first dealt 

 with. In the simplest forms, such as the 

 common sea - anemone, there are no 

 spicules at all. but the body wall is ren- 

 dered more or less consistent by the 

 development of fibrils of connective 

 tissue in the mesoderm ; this may be 

 called the supporting lamella, and, as we 

 it is thinner in the tentacles than in 

 the rest of the body, where it may become thrown 

 into folds ; from the body wall bands or septa, in 

 the midst of which is a more or less thin support- 

 ing lamella, project inwards, and some of them reach 

 the wall of the gastric cavity which lies in the central 

 axis of the polyp. (See Fig. 54.) In rare cases the 



Fig. 115. Hya* 

 lonema sie- 

 boldi. (After 

 Schultze.) 



may suppose, 



