INTEGUMENT. 



17 



Animals living in the water mostly possess a thinner horny layer which is 

 more capable of imbibition than that of land animals, which latter are usually 

 exposed to more mechanical dangers than the former. It may also be 

 mentioned that the connective-tissue bundles in the derma of Fishesj 

 Amphibians, and Reptiles show a typical arrangement in alternating 

 horizontal and vertical strands. Their disposition in Birds and Mammals is 

 irregular, that is, the ribres are more thoroughly felted. 



Fishes. In Amphioxus, the surface of the epidermis is 

 covered with cilia in the larval (gastrula) condition, and this 



Jf Jt 



Co 



FIG. 10. DIAGRAMMATIC TRANSVERSE SECTION ILLUSTRATING THE STRUCTURE 



OF THE SKIN IN FISHES. 



Ep, epidermis ; Co, derma ; F, subcutaneous fat ; OS, cuticular margin ; B, B, slime- 

 cells (goblet-cells which open on the surface) ; Ko, goblet-cells ; Ko, granular 

 cells present in Petromyzon ; G, vessels which pass upwards in the vertical con- 

 nective-tissue bundles of the derma ; W, horizontal connective-tissue bundles. 



must undoubtedly be considered as inherited from Invertebrate 

 ancestors. The striated cuticular border of the outer epidermic 

 layer in many fishes (e.g. Cyclostomes, Teleostei, and Dipnoi), 

 and, as will be mentioned presently, in Amphibian larvee, is perhaps 

 to be explained in the same wav, the striation possibly corresponding 

 to coalesced cilia (Figs. 10 and llA, CS}. 



In Amphioxus and the Cyclostomes long cylindrical cells provided with 

 stiff bristle-like processes appear amongst the ordinary epithelial cells. These, 



C 



