TEGUMENTARY ORGANS— SKIN STRUCTURES. 469 



plate forms radiating upgrowths by the infolding from 

 below of the epiderm and deposit of calcareous matter 

 in the folds (Fig. 329); (2) the outer margins of the 

 radiating calcareous septa unite to form the outer wall 

 of the theca; (3) the body wall by downfolding grows 

 over this outer wall, inclosing and covering it and add- 

 ing to it by calcareous deposit, and thus completing the 

 outer surface (Fig. 330). 



SPONGES. 



The skeletal deposits in sponges also are formed by 

 the agency of unmodified cells, and therefore may be 

 brought under this head. What we call a sponge is the 

 horny skeleton of the animal of the same name, and 

 deposited within its tissues in the most intricate way. 



Fig. 331. — Shells of living foraminifera : A, textularia variabilis; B, pene- 

 roplis planatus; C, rotalia concamerata. (After Williamson.) The 

 figures are greatly enlarged. 



The glass sponge, such as the Venus's flower-basket — 

 Euplectella — with its beautiful and intricate mesh of 

 glass fibers, is similarly produced by a deposit of silica 

 in the tissues of the animal. The classification of 

 sponges is based on the character of the skeleton. 

 There are horny sponges, glass sponges, and calcareous 

 sponges. 



