160 



ESSENTIALS OF ZOOLOGY 



arch, is formed by the co-operation of haemal spines. The 

 haemal arch encloses the caudal artery and vein. 



That portion of the vertebral column from the region of 

 the pectoral girdle forwards to the skull is converted into a 

 single mass of cartilage perforated merely for the roots of the 

 spinal nerves. In that part of the vertebral column also, as in 

 the skull in front of it, the notochord is reduced to practical 

 if not complete extinction. This change is to be regarded as 

 being associated with the relativejimmobility of that part^of 

 the body. 



Ant. end of 

 hind.b. 



Sphenoid ?*wb> 



Midb. 



Post. end of 



'r. arch 4 



PineaV 

 process 



Foreb. 



FIG. 79. Acanthias. The early condition of the 'chondrocranium. The 

 cartilaginous vertebral column, parachordals, trabeculae, and sphenoid 

 plates are indicated by dots, the visceral arches by heavier shading. 

 The nerves are shown and numbered. Modified after Sewertzoff . 



The skull consists of a brain case, or chondrocranium, 

 developed round the brain, and a series of arches developed 

 around the anterior end of the alimentary canal. 



The chondrocranium originates in a pair of cartilaginous bars, 

 one on each side of the notochord : these are the parachordals ; 

 and in a pair of cartilages which extend forwards from the 

 anterior end of the notochord, the trabeculae. As the front 

 end of the head is bent downwards at this stage, these elements, 

 to start with, are placed at about right angles with one another 

 (fig. 79). The angle between them is marked by the position 

 of the infundibulum of the brain. With the appearance and 

 expansion of the parachordals and trabeculae another pair of 



