532 PISCES FISHES. 



obvious features. On the dorsal surface the skin is pig- 

 mented 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 separated 

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

 tures, 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, if not wholly, due to the ectoderm 

 (epidermis), the rest to the mesoderm (dermis) ; the whole 

 arises as a skin papilla. The enamel is practically in- 

 organic, the cells having been replaced by lime-salts ; 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 

 a small electric organ in the tail region of Raja batis and 

 R. clavata^ apparently too small to be of any use, probably 

 incipient rather than vestigial. 



Electric organs are best developed in two Teleostean fishes a S. 

 American eel (Gymnotus] and an African Siluroid (Malapterurus), and 

 in the Elasmobranch Torpedo. In Gymnotus they lie ventrally along 

 the tail, in Malapterurus they extend as a sheath around the body, 

 and in Torpedo they lie on each side of the head, between the gills and 

 the anterior part of the pectoral fin. In other cases where they are 



