76 STRUCTURES OF SHARKS 



P and M. The second arch serves as the principal support 

 of the jaw hinge, HM, while holding in position, ventrally, 

 the hinder arches; it also supports the tongue, and forms 

 the hinder border of the spiracle (p. 19). The succeeding 

 arches, usually five in number, are the bearers of the func- 

 tional gills, their jointed structure permitting the dilating 

 and contracting movements of breathing. 



As a further skeletal element of the Elasmobranchs the 

 sub-notochordal rod is to be mentioned. It is present in 

 the larval stages of sharks, and appears to persist in the 

 adult Cladoselache (p. 79). It is a prominent structure 

 of the hinder body region, passing along, like a second 



notochord, immediately below the 

 vertebral axis. Its significance is 

 unknown. 



II. The INTEGUMENT of the 

 sharks, as has been noted (p. 23), 

 is studded with shagreen denticles, 

 Fi S- 85. Vertebrae of often in metameral arrangement. 



shark (Squatina) , longitudinal 



section. (After ZITTEL ) These have been shown to corre- 



"pond dearly with the teeth. 



centrum. iv. Intervertebral The Soft Structures characteristic 



space, w. Centrum. . 



of the Elasmobranchs include : 



III. GILLS, arranged metamerally (p. 19) ; the most 

 anterior one partly functional in the spiracle, SP. 



IV. SENSE ORGANS OF THE LATERAL LINE, in some 

 forms in an open sensory groove, in others sunken and 

 constricted in metameral pouches. 



V. BRAIN, simple in its segmental characters and 

 cranial nerves (v. p. 274). 



VI. NASAL ORGAN, EYE AND EAR, as shown on p. 276. 



VII. RENAL AND REPRODUCTIVE SYSTEMS (p. 270), ab- 

 dominal pores (p. 271). 



