INTEGUMENT OF FISHES 135 



The corium is coiitimious with a region of looser fibroelastic 

 connective tissue, the suhcutis, which in turn continues with the 

 deeper connective tissue sheaths surrounding the voluntary muscles 

 of the skeleton. Adi])ose tissue appears in relatively large amounts 

 in the subcutaneous connectixe tissues of most vertebrates. There 

 are marked variations in the details of skin structure in the different 

 classes of animals. 



INTEGUMENT OF FISHES. 



The epidermis of a fish is usually eom])osed of stratified squamous 

 epithelium. Low columnar or cuboidal cells occur in the stratum 

 germinativum, polyhedral cells compose the intermediate layers, 

 and squamous cells form the superficial layers. Unicellular and 

 multicellular glands secrete mucus, which forms a protective film 

 over the surface. Scattered in the lower portion of the epidermis 

 are groups of epithelial cells forming simple sensory organs which 

 receive free nerve endings. 



The dermis is relatively thin and is composed of fibrous connec- 

 tive tissue lying i)arallel to the skin surface. Pigment cells are 



Enamel 



n t- „ ■ '■ ' ■' ' '•' ' •' • '•'- 'dr^yy/ ^^:';:: .■ :■'-: --rri — Epidermis 

 Dentine ^ — Tisrirn^l^'^'t^Jr^ f-»l. _- ' L 



^,- ^ _ ... __.^^^. -Corium 



Pulp (cor in m)- 



FiG. 81. — Diagram of structure of a placoid scale of an elasmobranch. The enamel 

 develops from the epidermis; the dentine from the corium. 



abundant in this region. In some fishes the integument is com- 

 posed simply of the skin and its glands, but in the majority there 

 are additional structm-es, the scales, which are formed mainly from 

 outgrowths of the cor i inn. 



Scales. — In the P^lasmobranchs, multiplication of dermal cells 

 occurs at regular inter^'als to form a small papilla which projects 

 above the corium and carries along the stratum germinativum of the 

 epidermis. The peripheral cells of these papillae with the adjacent 

 dermal cells become osteoblasts and secrete a bony plate co\ering 

 each papilla. The o\erlying epidermal cells become active and 

 deposit a layer of enamel over this bony dentine base to form the 

 placoid type of scale. With continuous growth, spine-like papillae 

 project from a fiat base in the corium above the general epidermal 

 covering. The central portion of these scales is filled with connec- 

 tive-tissue pulp, which carries blood vessels and nerves. (Fig. 81.) 



