35G 



COMPARATIVE ANATOMY 



sacs. 



gill-cleft known as the spiro.dc (p. 88), is nearly always present 

 more anteriorly, between the mandibular and hyoid arches. In 

 the Holocephali the spiracle is wanting, and there are only four 

 clefts and three holobranchs in addition to hemibranchs on the 

 hyoid and fourth branchial arch : moreover, an opercular membrane 

 is present, covering the external branchial apertures and opening 

 by a slit posteriorly. In Chlamydoselachus fringed folds from the 

 hyoid and interb ranch ial septa project over the clefts. 



In Ganoids and Teleosts there are no longer chambered gill- 

 The septa on which the gill-lamina 3 are borne become 

 greatly reduced, so that the apices of the 

 latter extend freely outwards ; the whole 

 branchial region is, moreover, covered over 

 by the operculum and branchiostegal 

 membrane (cf. pp. 89 and 94), and thus, 

 as in the Holocephali, the gill-slits open 

 into a common branchial chamber, which 

 communicates with the exterior by a 

 single slit-like aperture on either side 

 (Figs. 261, B, and 262). A spiracle is 

 present in Acipenser, Polyodon, and Poly- 

 pterus amongst Ganoids. 



The mechanism of respiration in 

 Teleosts is as follows. In inspiration, an 

 expansion of the oral cavity takes place 

 by the opercular apparatus being raised, 

 the branchiostegal membrane at the same 

 time moving inwards so as to close the 

 opercular slit. An elastic, valve-like fold 

 of the mucous membrane, enclosing 

 numerous smooth muscle-elements, is 

 present in the maxillary region projecting 

 downwards from the roof of the mouth, 

 and a similar fold arises from the floor 

 of the mouth in the mandibular region. 

 On the expansion of the oral cavity and 

 closure of the branchiostegal valve, the 

 pressure of the water causes the maxillary 

 and mandibular valves to open inwards, 



and thus to admit the inspiratory current (Fig. 263, A). The 

 movements of expiration then follow' by the contraction of the 

 opercular apparatus, which causes the water in the mouth to press 

 on the maxillary and mandibular valves and thus close them, 

 while the branchiostegal valve is opened (B). Thus the mechanism 

 of these valves is quite similar to that of the valves of the heart, 

 and the respiratory current is produced by the contraction of 

 the walls of the mouth, which act like a pump. 



FIG. 262. TRANSVERSE 

 SECTION* THROUGH A 



HOLOBRANCH OF Zl/'J ii't 



(ox THE RIGHT) AND 

 (Indus (ON THE LEFT). 

 SLIGHTLY ENLARGED. 

 (From R. Hert\vig's 

 Zooloi/y. ) 



a, afferent, and i; efferent 

 branchial vessels ; b, 

 branchial arch : lif 1 , an- 

 terior, and W-, posterior 

 heniibranch of the gill; 

 /(, septum ; r, cartilagin- 

 ous gill-ray ; z, gill- 

 rakers. 



