324 CHORDATA. 
We have already referred to the exoskeleton of scales 
and teeth, but a more extensive endoskeleton has to be 
noticed. This is entirely formed in the meso- 
blast and consists of connective tissue (or mem- 
branous tissue) and cartilage. The connective tissue binds 
all the organs together and may be directly compared with 
that of Amphioxus. A gentle heat serves to disintegrate 
this tissue and enables us to easily isolate the firmer and 
Skeletal. 
Fig. 231.—DorsaL VIEW OF CRANIUM OF A SKATE. (4d nat.) 
Rostrum. 
Anterior Fontanelle. 
Orbito-nasal Foramen. 
Olfactory Capsule. 
Posterior Fontanelle. 
Orbit. 
‘Auditory Capsule. 
Auditory Aperture. _ Foramen Magnum. 
more consolidated cartilage. - In certain. parts the cartilage 
is hardened by the deposition of calcareous matter, a fore- 
shadowing of the “ bone ” of other forms. 
For purposes of description we may divide the cartila- 
ginous skeleton into (1) Axial and (2) Peripheral (appendic- 
ular). The Axial is divided into (1) Cranium, (2) Visceral 
arches and (3) Vertebral column; and the Peripheral into 
(1) Pectoral and (2) Pelvic elements. 
1. AxtaL.—The Cranium is an elongated hollow case 
enclosing a spacious cavity in which lies the brain. At the 
