SCALES. 53 



The epidermis contains large mucous cells which discharge 

 their contents on tlie surface. It may also contain pigment 

 cells and leucocytes. In many fishes the slime which is excreted 

 by the skin is poisonous. 



The skin is seldom completely without skeletal structures 

 {Marsipobranchii). As a rule scales, formed as ossifications 

 of dermal papillae which are typically completely covered by 

 the epidermis, are embedded in it. Fish scales * are of three 

 principal kinds : (1) Placoid scales which consist of small 

 plates of bone in the dermis carrying an upstanding spine which 

 projects freely, and is formed of dentine capped with enamel. 

 These are found in Elasmobranchii and some Ganoids. (2) 



Fig. 32. — Perca fluviatiiis (R^gne animal). 



Ganoid scales are bony plates covered with a smooth layer of 

 a substance called ganoin. Ganoin f is a dermal product 

 allied to vitro-dentine. These scales are entirely dermal, and 

 if their surface is exposed, it is owing to the fact that the epidermis 

 has been rubbed off. Such scales are found in most Ganoids. 

 (3) Scales of varying thickness consisting of bone only, without 

 ganoin. They are found in most Teleosteans, and are there 

 called cycloid and ctenoid scales according to the nature of their 

 edges. 



The unpaired fins arise as a continuous fold of skin extending 



* A fuller accovint of the scales is given with the accounts of the orders. 

 For principal recent literature, see Klaatsch, Morph. Jahrb. IB, p. 258, 

 and 21, 1894. p. 153; F. Maurer Die Epidermis u. ihre Abkommlinge, Leip- 

 zig, 1895. O. Hertwig, Morph. Jahrb., ii. and vii. C. Rose, Anat. Anz., 

 U, 1897 ; pp. 21 and 33. Nickerson, " The development of the scales of 

 Lepidosteus," Bull. Mus. Harvard, 24, 1893. 



t It was formerly svipposed to be enamel and to be epidermal in origin, 

 but this has been proved to be erroneous. 



