346 MANUAL OF ELEMENTARY ZOOLOGY 



the centrum appears as a biconcave disc. On each side 

 the centrum bears a pair of ventrilateral processes^ In the 

 trunk region these are directed outwards and bear short 

 ribs, which lie beneath the muscles of the back; in the 

 hinder part of -the body the processes are directed down- 

 wards and are known as hizmal arches, enclosing a hamal 

 canal, in which lie the caudal artery and vein. Towards 

 the hinder end of the tail they fuse at their ends and bear 

 a median hamal spine. Between the neural arches of 

 successive vertebrae are wide gaps which are closed by 

 intercalary pieces. The neural spines axe. a series of fiat 

 median pieces of cartilage, twice as numerous as the verte- 

 bras, which fill the gaps between the tops of the neural arches 

 and intercalary pieces and roof in the vertebral canal 



The skull consists, like that of the frog, of a cranium 

 which contains the brain, with a pair of nasal 

 capsules in front, a pair of auditory capsules 

 one at each side of the hinder end, and a visceral skeleton 

 below. The nasal capsules are large, thin-walled structures, 

 continuous with the cranium, widely open below, and sepa- 

 rated by the cartilaginous internasal septum or mesethmoid 

 cartilage. Three slender processes, one from the front wall 

 of each capsule and one from the mesethmoid cartilage, 

 project into the snout and are together known as the 

 rostrum. At the junction of the cranium and the nasal 

 capsules the roof of the skull shows a large gap, the anterior 

 fontanelle. From the sides of the cranium large supra- and 

 suborbital ridges project to protect the orbit above and below. 

 On the auditory capsules, which are continuous with the 

 cranium, ridges mark the position of the semicircular canals. 

 A median depression on the roof between the auditory 

 capsules communicates on each side with a canal, carrying 

 the aqueductus vestibuli, which leads to the internal ear and 

 puts the endolymph into communication with the sea water 

 through a small opening. There is no ear drum. At the 

 hinder end, between two occipital condyles, may be seen the 

 notochord, which passes into the floor of the cranium for 

 some distance. Numerous openings pierce the wall of the 



1 These are often called transverse processes, but they do not corre- 

 spond with the transverse processes of the frog, which belong to the 

 neural arches. 



