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FISHES 



not from it but around it. The tissue surrounding the noto- 

 chord grows inwards at regular intervals so as to form rings 

 which press into and constrict the notochord. The substance of 

 these rings develops into cartilage, and the rings themselves be- 

 come the bodies or " centra " of the vertebras, which are hollow 

 at each end like a dice box, so that the gelatinous substance of 

 the notochord persists in the adult dog-fish between the " centra ". 

 From the centra dorsally (Fig. 18) there extend processes 

 called neural arches which enclose the spinal cord, and between 

 the neural arches are intervertebral plates ; in the middle line 

 above are small pieces called neural spines ; at the sides ven- 

 trally in the abdominal region of the body the centrum projects 



a.c. i.p. n.a. 



Fig. 18. — Skull and gill-arches of Dog-fish (Scyllium canicula) with anterior 

 part of vertebral column, n.a., neural arch; i.p., intervertebral plate; o.c, 

 olfactory capsule; a.c, auditory capsule; «./., upper jaw; l.j., lower jaw; hy., 

 hyoid arch ; br. 5, fifth branchial arch. 



into transverse processes, attached to which are slender ribs. 

 All these parts are composed in the dog-fish of cartilage, to 

 some degree hardened by the deposit of lime, but not having 

 the structure of true bone ; in fact all the internal skeleton 

 of this group of fishes is cartilaginous, whereas in other fishes 

 it is replaced by bone. Attached to the anterior end of the 

 vertebral column is the skull which consists of an undivided 

 capsule of cartilage ; in it can be distinguished the brain 

 case in the median position, and two pairs of capsules at the 

 sides, the anterior pair enclosing the olfactory organs and 

 the posterior pair the auditory organs ; at the sides between 

 these capsules the skull is hollowed out to form the orbits in 



