INTEGUMENT 



147 



of bone which are formed, not in the epidermis, but in the 

 dermis. In some fishes there is deposited on the upper surface 

 of the bony scale a layer of enamel which is due to the activity 

 of the cells of the overlying epidermis. Teeth are also formed 

 in this way, the dentine being merely a kind of bone. Antlers 

 are bone and are formed by the dermis. 



349. Glands. — Glands form another type of differentiated 

 integumentary structures. They are of the simplest form in 

 hydra and worms where they consist of single cells of the 

 epidermis. These cells do not secrete a cuticula on their free 



Co. 



Ep. 



Oil 





g- 



Fig. 71. — Glands. A, The ectoderm of hydra showing granules of cement (g) 

 secreted by the cells at the surface; B, a section through the epidermis of nereis, 

 showing a number of unicellular glands. Only the outhnes of the outer ends of 

 the gland cells are shown. The nuclear portions are below but not distinguish- 

 able in the figure. Cti, Cuticula; Ep, epidermis; g, pores of the glands; C, 

 diagram of a unicellular gland at the beginning of secretion; D, the same when 

 swollen with secretion; E, the same cell after its contents are ejected. 



surfaces, but do secrete other substances which at first accumu- 

 late within the bodies of the cells but are later forced out 

 through the pores in the cuticula which were formed by the non- 

 secretion of cuticula by the gland cells. The secretions accumu- 

 late until the excess gradually oozes out through tlie pores or is 

 suddenly forced out in larger quantities by the contraction of the 

 surrounding tissues. In the case of worms and many other 

 aquatic animals the substance secreted takes up water and forms 

 sHme. The function of the slime is probably in most cases 



