THE SKELETON OF THE DOGFISH. THE SKULL. 73 



for the exit of the dorsal or sensory roots of the spinal 

 nerves. 



The neural spines are small patches of cartilage filling 

 up the gaps between the dorsal ends of the neural plates. 



The haemal arches (basiventralia) differ much in the 

 trunk and tail portions of the vertebral column. In the 

 trunk portion the centra are flattened below, and the two 

 halves of the haemal arch diverge from one another as blunt 

 ventri-lateral processes to which short cartilaginous rods, 

 the ribs, are attached. Further back at about vertebra 37, 

 the two halves of the haemal arch project downwards and 

 meet forming a complete arch. Further back still, towards 

 the hind end of the tail, the haemal arches bear median 

 haemal spines (ventrispinalia). 



B. THE SKULL. 



The skull of the dogfish remains cartilaginous throughout 

 the life of the animal, and has consequently a far more simple 

 structure than have the skulls of higher animals, in which 

 complication has been produced by the development of bone. 



The skull consists of the following parts : 



(1) a dorsal portion, the cranium, which lodges the brain, 

 and to the sides of which the capsules of the auditory and 

 olfactory sense organs are united. The cranium may be 

 compared to an unsegmented continuation of the vertebral 

 column ; 



(2) a number of ventral structures, disconnected or only 

 loosely connected with the cranium. These together con- 

 stitute the visceral skeleton forming the jaw's and sup- 

 porting the gills. 



(1) THE CRANIUM. 



The Cranium is an oblong box, with a flattened floor and 

 a more irregular roof. Its sides are expanded in front owing 



