314 



HITCHCOCK'S ANATOMY 



scales are sometimes disposed in an imbricated manner upon 

 the body, and are not situated in the epidermis, "but really 

 in the skin, and included by it." The scales contain both 

 phosphate and carbonate of lime as a rule. Their shapes are 

 various, though Professor Agassiz, the best living authority 

 on fishes, has grouped all bony fishes under four orders, de- 

 pendent mainly upon the form of the scale. First, the Pla- 

 coidians, including such fishes as the Sharks and Rays, where 

 the scales are either large and covered with bony tubercles, 

 or simply with small enameled scales. Second, the Ganoi- 

 dians, including the Sturgeon and bony Pike, which have 

 angular bony plates coated with a thick layer of enamel. 

 Third, the Otenoidians, such as the Perch, which have hard 

 scales jagged on the outer edges, like the teeth of a comb. 

 Fourth, Cycloidians. These have soft and circular scales with 

 simple margins. In this order are found the Herring and Sal- 

 mon. Sometimes the scales are provided with a hook-like 

 process, which overlaps and fastens itself into a depression in 

 the scale beneath. 



559. Tegument of Articulata.— Horny Case of In- 

 sects. — The Tegumentary envelop of the sub-kingdom Arti- 

 culata has already been described under Comparative Oste- 

 ology, since the skin of many of this sub-kingdom, especially 

 the Crustacea and insects, is in reality the only skeleton these 

 animals have. Insects have a covering sometimes leathery 

 and soft, and sometimes horny and solid, which contains a 

 peculiar proximate (chemical) principle known as Chitine. 



560. Mantle of Molluscs, — In Molluscous animals, a 

 dermis, which is a muscular skin, envelops all the viscera, 

 and hence is called the cloak or mantle, wdiich secretes the 

 hard calcareous covering known as the shell. Generally the 

 shell is external to the mantle, as in the Clam and Oyster, 



What are the four orders dependent upon the form of the scale? Who is the author 

 of this arrangement ? Give examples of each. 559. What is the covering of the articu- 

 lata? What is peculiar about its chemical composition ? 560. Describe the outside cov- 

 ering of molluscs. 



