THE SKATE. 483 



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, though not (it seems) wholly due to the 

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

 cutis) ; the whole arises as a skin papilla. It may be noted 

 that enamel is practically inorganic, the cells having been 

 replaced by lime-salts, that dentine has 34 per cent, of 

 organic matter (apart from water), and that bone is a definite 

 cellular tissue. On the ventral unpigmented or less pig- 

 mented 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 

 rudimentary electric organ in the tail region of Raja batis 

 and R. clavata, apparently too incipient to be of any 



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 slightly 

 developed (certain 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 cartila- 

 ginous, but here and there ossification has begun, as a 

 crust over many parts, more deeply in the vertebrae, teeth, 

 and scales. 



The vertebral column consists of an anterior plate not 

 divided into vertebrae, and of a posterior series of distinct 

 vertebrae. Each of these has a biconcave or amphiccelous 

 centrum. From each side of the centrum a transverse 



