142 



ZOOLOGY 



SECT. 



leads into the large pharynx. Five pairs of slits running vertically 

 on each side of the neck the branchial slits also lead internally 

 into the pharynx. A large median opening on the ventral surface 

 at the root of the tail, between the pelvic fins, is the opening leading 

 into the cloaca, or chamber forming the common outlet for the 

 intestine and the renal and reproductive organs. A pair of small 

 depressions, the abdominal pores, situated behind the cloacal open- 

 ing, lead into narrow passages opening into the abdominal cavity. 



The skeleton is composed entirely of cartilage, with, in certain 

 places, depositions of calcareous salts. As in Vertebrates in general, 

 we distinguish two sets of elements in the skeleton the axial set 

 and the appendicular, the former comprising the skull and spinal 

 column, the latter the limbs" and their arches. 



n.a 



n.s.p 



n.c 



n.a, 



h.a 



FIG. 822. Portions of the vertebral column of Scyllium canicula. A and B, from the 

 trunk ; C and 7), from the middle of the tail ; A and C, two vertebrae in longitudinal section ; 

 B and D, single vertebrae viewed from one end. b, calcined portion of centrum ; c. centrum ; 

 for. foramen for dorsal, and for', for ventral root of spinal nerve ; h. a. haemal arch (basi- 

 ventral) ; h.c. haemal canal ; h.sp. haemal spine ; i.n.p. intercalary piece (interdorsal, 

 or interneural plate) ; n.a. neural arch ; n.c. neural canal ; n.p. neural plate (basi-dorsal) ; 

 n.sp. neural spine ; ntc. intervertebral substance (remains of notochord) ; r. proximal portion 

 of rib ; tr.pr. transverse process (basal stump). (From Parker's Practical Zoology.) 



The spinal column is distinguishable into two regions the 

 region of the trunk and the region of the tail. In the trunk-region 

 each vertebra (Fig. 822, A and B) consists of a centrum (c.), neural 

 arch (n.a.), and transverse processes (tr.pr). In the caudal region 

 there are no transverse processes, but inferior or hcemal arches 

 (C, D, h.a.) take their place. The centra of all the vertebrae are 

 deeply biconcave or amphicodous, having deep conical concavities 

 on their anterior and posterior surfaces. Through the series of 

 centra runs the notochord (ntc.), greatly constricted in the centrum 

 itself, and dilated in the large spaces formed by the apposition 

 of the amphiccelous centra of adjoining vertebrae, where it forms a 



