IX SKELETON 317 



lowly organized vertebrates is that throughout the greater part of the 

 vertebral column there are two neural arches — an anterior and a posterior 

 — corresponding to each centrum. In other words in a given length of 

 vertebral column there are twice as many neural arches as there are 

 centra. This appears to be the primitive condition but in the more 

 highly evolved vertebrates the anterior arch has apparently disappeared 

 leaving only a single neural arch corresponding to each centrum. 



In the tail region the haemal arch rudiments behave very much as 

 those of the neural arches, complete haemal arches being formed, pro- 

 longed into haemal spines. Further forward on the other hand, where 

 the small space enclosing the caudal artery and vein is replaced by the 

 large peritoneal cavity, the arch rudiments never meet ventrally but 

 merely spread out some distance in the body wall. Their terminal 

 portions become segmented off to form the ribs while their basal portions 

 continuous with the centrum are termed transverse processes. 



A necessary property of the vertebral column is its flexibility, so as 

 not to interfere with the flexure of the body from side to side in swimming, 

 and this flexibility is ensured by its being segmented up into the individual 

 vertebrae. When traced forwards towards the head region, where the 

 presence of the brain demands rigidity instead of flexibility, the segmenta- 

 tion of the vertebral column becomes more and more obscured. The 

 brain is enclosed and protected in a cartilaginous cranium (Fig. 133, Cr), 

 complete as regards its floor and side-walls but incomplete dorsally. 

 The floor represents a continuation forwards of the series of vertebral 

 centra, while the side walls in at least the hinder part represent a con- 

 tinuation forwards of the series of neural arches, but in so far at least 

 as the adult Dogfish is concerned practically all trace of demarcation of 

 the individual arches has disappeared. 



At the sides of the vertebrate head are the three pairs of special sense- 

 organs — the olfactory organ or nose, the eye, and the otocyst or ear — 

 and each of these is enclosed in a protective capsule of cartilage. In 

 the case of the two sense-organs which do not require to be freely movable, 

 namely the olfactory organ and the otocyst, their protective capsules are 

 firmly fused with the cartilaginous cranium — the olfactory capsule (Fig. 

 133, olf) in front and the auditory capsule (Fig. 133, aud) behind. 



In addition to the true cranium with its sense-capsules there is present 

 in the head region a characteristic arrangement of skeletal structures 

 embedded in the wall of the buccal cavity and pharynx. These form 

 the skeleton of the visceral arches — the masses of tissue lying between 

 the successive visceral clefts. Primitively this skeleton consists of a 

 series of hoop-like rods of cartilage embedded in the visceral arch near 



