^jC^ the dogfish, phylum chord ATA 



bears a pair of ventrilateral processes (Figs. 239, 246) which 

 are often called tranverse processes, though they do not corre- 

 spond with the transverse processes of the rabbit, which belong 

 to the neural arches. 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 downwards and meet to form haemal arches, which enclose 

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

 the hinder end of the tail the ventrilateral processes fuse at their 

 ends and bear a median hsemal spine. Between the neural arches 

 of successive vertebrae are wide gaps which are closed by inter- 

 calary pieces. A series of fiat median pieces of cartilage, the 

 supradorsals, twice as numerous as the vertebrae, fill the gaps 

 between the tops of the neural arches and intercalary pieces, 

 and roof in the vertebral canal. 



THE SKULL 



The skull is built on the usual vertebrate plan, but differs 

 from the skulls of the other types described in this book in being 

 made entirely of cartilage (p. 517). There is a dorsal part in 

 association with the brain, and a ventral part in association 

 with the gill shts. The dorsal part, to which the term skull is 

 sometimes restricted, consists of the brain-box or cranium, and 

 attached to this a pair of anterior olfactory capsules and a pair 

 of posterior auditory capsules. Optic capsules have a transient 

 existence in the embryos of vertebrates, but generally disappear 

 except in so far as they contribute to the sclerotic coat of the eye. 

 This disappearance may be connected with the mobility of the 

 eyes, which makes a fixed protective covering impracticable ; 

 instead, the eyes are sunk into hollows of the cranium called 

 orbits. The projections of the cranium above and below the orbits 

 are called supraorbital and suborbital ridges. In the anterior 

 part of the roof of the cranium there is a large gap ; such an area 

 of incompleteness in the cartilage of the cranium is called a 

 fontanelle, and this one, from its position, is the anterior fonta- 

 nelle ; others, in different positions, occur in other animals. 

 Through the fontanelle the brain may be seen. Posteriorly the 

 cranium has two knobs, the occipital condyles, which fit into 

 hollows on the first vertebrae. The nasal capsules, thinner than 

 l"'-:{^ rest of the skull, are continuous with the cranium, and are 



