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 vertebre, teeth, 
and scales. 
The vertebral column consists of an anterior plate not divided into 
vertebrze, 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 not only fills what would be a gap 
between the neural processes and the slightly developed neural spine, 
but also links the vertebrz together. In the caudal vertebree, what 
seem to be the transverse processes are directed downwards to fofm 
a heemal arch enclosing the caudal artery and vein. In the lozenge- 
shaped spaces between the vertebrz 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 
hheemal 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, the expanded bases of the arcualia fuse to form 
cartilaginous (eventually bony) centra, outside the sheath of the noto- 
chord. This is called an arczcentrous 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- 
ecule, sense capsules, etc. 
