112 STRUCTURE OF THE VERTEBRATES 



origin. Functionally all the ectodermal structures depend upon 

 the dermis for support, and as the blood vessels lie in the der- 

 mal layers the nutritive supply is also from these tissues. 



Phylogenetically, the earliest ectodermal structures are glands. 

 In the three most primitive classes (cyclostomes, fish and am- 

 phibia) there are no other structures of importance. In the 

 cyclostomes and fish these are mucous glands which keep the 

 skin clear of accumulated dirt, and are to a certain extent pro- 

 tective. In the amphibia the same functions are present, and in 

 addition most amphibia depend upon the mucous to keep the 

 skin moist, which is necessary for the skin's respiratory function. 



All glands of the vertebrates are based upon three simple 

 types: (1) single secreting cells; (2) tubular glands which are 

 straight or coiled impocketings of the ectoderm; and (3) acini- 

 form glands which are flask-shaped, the neck being the duct to 

 the outside and deeper part the acinus. Either of the last two 

 types may be branched, (1) as simple branches, or (2) into 

 highly complex structures. 



Functionally all glands go back to the individual, secreting, 

 epithelial cell. These cells are modified for secretory activity; 

 and the most complex gland is merely a collection of cells spe- 

 cialized for the same function. The duct serves as an outlet, 

 the lining cells being flattened and not secretory. 



Reptiles. The reptile skin is almost free of glands. A few 

 buccal, or mouth, glands are present; and most reptiles have 

 scent glands in the anal region. In a number of turtles and 

 snakes the odor is so distinctive that the animal can be de- 

 tected and identified without seeing it. 



Ectodermal scale _^^ Dermal plate 



,." Epidermis 



Papilla Alligator Skin (Long. Section) 



tizardSkin (Long. Section) with dermal scale embedded in 



the skin. 



Fig. 49. Sections through Reptile Scales. Reptiles are characterized by 

 ectodermal scales, although in some forms these are supported by 



dermal plates. 



