324 



C HO R DATA. 



We have already referred to the exoskeleton of scales 

 and teeth, but a more extensive endoskeleto7i 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 



Fig. 231. — Dorsal View of Cranium of a Skate. {Ad nat.) 



Rostrum. 



Anterior Fontanelle. 

 Orbito-nasal Foramen. 



Olfactory Capsule. 



Posterior Fontanelle. __4JE^^^B m\ ^ -Orbit 



Auditory Capsule. 



Auditory Aperture. Foremen 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 (i) Axial and (2) Peripheral (appendic- 

 ular). The Axial is divided into (i) Cranium, (2) Visceral 

 arches and (3) Vertebral column ; and the Peripheral into 

 (i) Pectoral and (2) Pelvic elements. 



I. Axial. — The Cranium is an elongated hollow case 

 enclosing a spacious cavity in which lies the brain. At the 



