146 ANIMALS 



344. This type of integument is extremely flexible and there- 

 fore does, not impede locomotion or other movements. At 

 the same time it is very tough, fairly resistent to mechanical 

 injury, and the lifeless superficial layers of the epidermis are 

 impervious to water and thus protect the living parts from the 

 dry air. 



345. In addition to these undifferentiated portions of the 

 various types of integument there are also certain important 

 specialized structures developed which serve as supplementary 

 protective organs, as organs of defense and offense, as prehensile 

 organs and as accessory organs of locomotion. 



346. The most common type of differentiation consists simply 

 of a local thickening of the cuticula or epidermis. Thus in 

 many worms minute tubercles or larger jaw-like structures are 

 found on the walls of the mouth and pharynx. The setae and 

 aciculae on the parapodia and sometimes scales on the back, are 

 of similar origin. In insects and Crustacea the sensory hairs 

 found especially on the antennae and mouth parts, and around 

 the joints of the appendages and body are also produced by the 

 unusually active secretion of chitin by one- or a few cells of the 

 underlying epidermis. 



347. In Vertebrates the epidermis becomes modified in a 

 great variety of ways by the aggregation of the minute horny 

 scales into exceedingly firm structures, which serve a great vari- 

 ety of purposes. Among the most important of these structures 

 are nails, claws, hoofs, spurs, horns, beaks, " tortoise shell," 

 " whale-bone," scales of certain kinds, hairs and feathers. The 

 scales found on reptiles, birds and some fishes are merely thick 

 and compact areas of the corneous layer of the epidermis. The 

 hair differs from the scale only in its form. The feather may be 

 likened to a hair greatly enlarged in diameter and hollow, with 

 certain parts of its shaft splitting in a complicated fashion and 

 thereby producing the vane. 



348. The scales of most fishes are not horn but thin plates 



