INTEGUMENT 



373 



as a series of alternate and overlapping plates, like the shingles on a 

 house. They push the posterior part of the connective-tissue pocket in 

 which they lie, up into the thick epithelium from which they are sepa- 

 rated by but a thin layer of connective tissue and their own matrix 

 cells. 



These matrix cells of the papilla, which were at first round and plump, 

 now become drawn out into a very thin epithelial-like layer. Those on 

 the upper side of the scale lay down as a rule a more uneven surface on 

 the scale than those in the lower layer. In many fishes the upper sur- 

 face, especially on its posterior part, is covered with many points, knobs, 

 or other processes which cause it to be called a ctenoid scale. During its 





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4& 



S^^^^Wiliii?-^- ; 



FIG. 341. Two scales near maturity in the skin of a young trout, b.m., basement membrane 

 of the overlying striated epithelium; conn.t., connective-tissue corium; /., scale follicle 

 formed from connective tissue of corium. The mesodermal formative cells lie on the distal 

 and proximal surfaces of the scales, forming a very thin layer on each surface. (After 



NUSBAUM.) 



earliest stages the young scale lies almost flat or parallel with the body 

 surface. As it grows in extent, the posterior end is tilted up. In Figure 

 341 the shrinkage of the layers has caused the scales to stand up much 

 more than they do in life. The ganiod and selachian fishes also have 

 mesodermal structures developed in the skin for protection. 



The integument of the echinoderms also shows a set of hard plates 

 consisting of a deposit of lime in its tissues. This series is another ex- 

 ample of the comparatively rare cases in which these mechanical pro- 

 tective structures are developed in the cutis. 



The epidermis in these animals is a thin, simple epithelium, and not 

 fitted to stand either abrasion or pressure. The underlying plates of the 

 cutis not only protect the body from pressure by their rigidity but they 

 also protect the epidermis from abrasion by the formation of outlying 



