478 FISHES. 



beside it. We may feel the pectoral and pelvic girdles 

 supporting the fore and hind fins. In the male, the latter 

 are in part modified into 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 latter being due 

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

 (dermis or cutis) of the skin, the whole arising as a skin 

 papilla. On the ventral unpigmented surface are numerous 

 mucus canals or jelly tubes, ending in ampullae. These 

 are also present on the dorsal aspect, especially about the 

 head. They have a sensory function. Most of the slime 

 which exudes on the surface comes 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 rudimentary electric 

 organ in the tail region of Raja batis and R. davata^ 

 apparently too incipient to be of any use. 



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, 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 animal. 



The Skeleton. 



The skeleton is for the most part cartilaginous, 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. 



