590 



PHYLUM CHORDATA : CLASS PISCES — FISHES 



merited and studded with placoid scales ; on the top of 

 the skull there are two unroofed areas or fontanelles ; 

 numerous jointed radials support the pectoral fins. Behind 

 the lidless eyes are the spiracles — the first of the obvious 

 gill-slits, opening dorsally, containing a rudimentary gill, 

 and communicating posteriorly with the mouth cavity. 

 On the ventral surface are seen the sensory mucus canals, 

 the transverse mouth, and the nostrils incompletely separ- 

 ated from it, as if in double harelip, the five pairs of gill 

 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 co- 

 pulatory " claspers." 



The skin. — On the dorsal 

 pigmented surface, em- 

 bedded 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 



£., Tip of enamel; IV., ivory or mainly, if UOt wholly, duC 



dentine; B., base of bone; EP., tO the CCtodcrm Cenidermis'J 

 epidermis, broken by the out- " ct^iuuciin \^cpiuciiiut>;, 



growth of the scale ; D., the dermis. the rCSt tO the mCSodcrm 



(dermis) ; the whole arises 

 as a skin papilla. The enamel is practically inorganic, 

 the cells having been replaced by lime-salts ; dentine 

 has 34 per cent, of organic matter (apart from water) ; 

 the bone is more obvious cellular tissue. On the ventral 

 unpigmented or less pigmented 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 be recognised. The large muscles which work the 

 jaws are noteworthy. Professor Cossar Ewart has described 



Fig. 331. — Diagram of the placoid 

 scale of an Elasmobranch fish. 



