THE SKELETON. 533 



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. 



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 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 net 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 haemal arch enclosing the caudal artery and vein. In the lozenge- 

 shaped spaces between the vertebrae 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 

 haemal 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 chordacentrous vertebral column. In Teleostomes and 

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

 cartilaginous (eventually bony) centra, outside the sheath of the noto- 

 chord. This is called an arcicentrous vertebral column. 



The skull is a cartilaginous case, with a spacious cavity 

 for the brain, a large posterior aperture or foramen magnum 

 through which the spinal cord passes, two condyles working 

 on the end of the vertebral plate, a large ear capsule on 

 each side posteriorly, a similar nasal capsule on each side 

 anteriorly, a long rostrum in front, two fontanelles on the 

 roof. Compared with the skull of a cod or of a higher 

 Vertebrate, that of a skate is simple ; it is not ossified, nor 

 divided into distinct regions, nor has it anything corre- 

 sponding to the investing membrane bones, which in higher 

 animals are added to the original foundations of the skull, 

 nor do the visceral arches in the skate take part in forming 

 the skull, which arises, as usual, from parachordals, trab 

 eculae, sense capsules, etc. 



