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organic, the cells having been replaced by lime-salts ; 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 datis 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 (Ala/apterurus), and 
in the Elasmobranch Zorpedo, 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 
