THE SKATE. 



5*1 



apertures, the cloacal aperture and two abdominal pores 

 beside it. Pectoral and pelvic girdles support the fore- and 

 hind- fins. In the male the hind-fins are in part modified 

 into complex copulatory " claspers." 



The skin. On the dorsal pigmented surface, embedded 

 in the dermis, there are many "skin-teeth," or "dermal 

 denticles," or " placoid scales." Each is based in bone, 

 cored with dentine or ivory, tipped with enamel. The 

 enamel is mainly, though not (it seems) wholly due to the 

 ectoderm (epidermis), the rest to the mesoderm (dermis or 

 cutis) ; the whole arises as a skin papilla. It may be noted 



V. 



R 



(in.) 



(IV.) 



D. 



L 



V. 



FIG. 244. Diagram offish forms in section. 



D., Dorsal ; V., ventral ; R., right ; Z,., Left. 



(I.) Typical section, e.g. haddock. 

 (II.) Flattened from side to side, but remaining vertical, e.g. John 



Dory. 

 (III.) Flattened from side to side, but resting and swimming on one 



side, e.g. flounder. 

 (IV.) Flattened dorso-ventrally, e.g. skate. 



that enamel is practically inorganic, the cells having been 

 replaced by lime-salts, that dentine has 34 per cent, of 

 organic matter (apart from water), and that bone is a definite 

 cellular tissue. On the ventral unpigmented or less pig- 

 mented surface there are numerous mucus canals or jelly 

 tubes, sensory in function. Some are also present on the 

 dorsal aspect, especially about the head. Most of the slime 

 exudes from glandular goblet cells in the epidermis. 



Muscular system. In the posterior part of the body and 

 in the tail, the segmental arrangement of the muscles may 



