4i6 FISHES. 



nostrils incompletely separated therefrom as if in double 

 hare-lip, the five pairs of gill apertures, the cloacal aperture 

 and two abdominal pores beside it. We may feel the 

 pectoral and pelvic girdles supporting the fore and hind fins, 

 or limbs. In the male, the latter have " claspers " which 

 are inserted into the cloaca of a female during copulation. 



Skin. — On the dorsal pigmented surface 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 latter being due to the ectoderm (epi- 

 dermis), the rest to the mesoderm (dermis or cutis) of the 

 skin, the whole being originally a skin-papilla. On the 

 ventral unpigmented surface, numerous mucous canals are 

 readily seen, and there are also some on the dorsal aspect. 

 These jelly-tubes have a sensory function. Most of the 

 slime which exudes on the surface comes from glandular 

 goblet-cells in the skin. 



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. In the tail-region. Prof. Cossar Ewart has 

 demonstrated a rudimentary electric organ in Raja batis and 

 R. clavata. 



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

 American eel ( Gymnotus) and an African Siluroid i^JWalaptei'urus), and 

 in the Elasmobranch T'orpedo. In Gymnotus they lie ventrally along 

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

 developed, both in Elasmobranchs and Teleosteans, they lie in the tail. 

 Separated from one another by connective tissue partitions, are numerous 

 "electric plates," which consist of strangely modified muscle-substance 

 and numerous nerve-endings. The electric discharge is very distinct in 

 the three forms noted above, and is controlled in some measure at least 

 by the will of the animal. 



The Skeleton is for the most part gristly, but here and 

 there ossification has taken place, as a crust over many 

 parts, but more deeply in the vertebral bodies, in the teeth, 

 and in the tooth-like scales. 



The notochord has an endodermic origin ; the enamel 

 tips of the teeth and skin-teeth are ectodermic ; otherwise 

 the entire skeleton is derived from the mesoderm. 



