SKELETON OF SKATE 



591 



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 Elasraobranch 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 cartil- 

 aginous, but here and 

 there ossification has 

 begun, as a crust over 

 many parts, more 

 deeply in the vertebrae, 

 teeth, and scales. 



Fig. 332. 



-Diagram of the " soft " scales 

 of a Bonv Fish. 



EP., Transparent epidermis extending over the 

 scale (B.) of vitrodentin ; D., the dermis. 

 One scale overlaps another, but more closely 

 than the diagram shows. 



The vertebral column 

 consists of an anterior 

 plate not divided into 

 vertebrae, and of a pos- 

 terior series of distinct vertebrae. Each of these has a biconcave or 

 amphicoelous centrum. From each side of the centrum a transverse 

 process projects outwards, and bears a minute hint of a rib. From 

 the dorsal surface of each centrum two neural processes arise. 

 Between each two vertebrae there is at each side a broad interneural 

 plate, which not only fills what would be a gap between the neural 

 processes and the slightly developed neural spine, but also links the 

 vertebrae together. In the caudal vertebrae, what seem to be the 

 transverse processes are directed downwards to form a h»mal arch 

 enclosing the caudal artery and vein. In the lozenge-shaped spaces 

 between the vertebra lie gelatinous remains of the notochord. 



In Selachians and Dipnoi amoeboid cartilage cells from the arcualia 

 (paired nodules of cartilage in the mesenchyme or embryonic connec- 

 tive tissue outside the sheath of the notochord, which form neural and 

 hcsmal arches) migrate into the sheath of the notochord and convert 

 it into a cylinder of cartilage (segmented into centra in Selachians). 

 This is called a chordacenirous vertebral column. In Teleostomes and 

 higher Vertebrates, the expanded bases of the arcualia fuse to form 



