76 STRUCTURES OF SHARKS 
Pand M. The second arch serves as the principal support 
of the jaw hinge, 1, 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, 
Fig. 85.— Vertebre of often in metameral arrangement. 
shark (Sguatina), longitudinal 
section. (After ZITTEL.) These have been shown to corre- © 
ch, Notochord. d. Calcified * 
rim and anterior surface of SP0nd clearly with the teeth. 
centrum. iv, Intervertebral The soft structures characteristic 
space. w. Centrum. . 
of the Elasmobranchs include :— 
III. Gitvs, 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). 
