GILLS 95 



generally gill-less. The way in which the gills and the visceral 

 arches become specialised is of great importance in classification. 



Primitively, no doubt, each gill-pouch opened independently 

 to the exterior,' as is still the case in most Elasmobranchs. The 

 region between consecutive gill-pouches became narrowed to a thin 

 septum bearing the gill -lamellae (Fig. 26). The skin, with its 

 exoskeleton, was still continuous along the outer edge of the 

 septa. 



In most fish, however, the free edge of the hyoid gill-flap, the 

 septum of the hyoid arch, grows backwards as an opercular flap 

 covering the branchial openings. Accompanying the development 

 of this operculum (Figs. 26, 57) is a reduction of the other septa, so 

 that the gill-lamellae come to project freely at their outer ends in 

 the branchial chamber. 



It is impossible as yet to decide what was the original number 

 of gill-slits and arches in the common ancestor of the Pisces. But, 

 since seven branchial slits and arches are found behind the hyoid 

 arch in some living Selachians (Heptanchus, p. 140), and traces of 

 a seventh arch may be identified in the larynx of Amphibia, 

 it seems probable that the early forms had at least eight 

 gill-slits (counting the spiracle). This conclusion is supported 

 by the discovery of vestiges of gill-clefts, behind the normal six, 

 in some living fish, and by the fact that the number of clefts 

 becomes progressively reduced from behind forwards in more 

 specialised forms (five branchial slits in most fish, four in 

 Holocephali, three in many Teleosts). 



The primitive jaws, derived from the first visceral arch (p. 18), 

 become directly or indirectly attached to the skull. In front, the 

 upper or palato-quadrate bar is generally movably connected with 

 the lateral ethmoid region of the skull by the palato-basal or 

 ethmoid process. Behind, the quadrate region of the bar may either 

 be directly fused to the auditory capsule (Dipnoi, Fig. 206), or it 

 may be supported away from the skull by the dorsal element of the 

 hyoid arch, the hyomandibular (Selachii, Teleostomi, Figs. 58, 59), 

 or again it may be connected with the auditory capsule both by an 

 articulation above and by the hyomandibular (Notidani, Fig. 59). 

 Huxley [230] named these three types of skull and jaw suspension 

 autostylic, hyostylic, and amphistylic respectively. In the autostylic 

 type the hyomandibular becomes much reduced, and may disappear. 

 In the hyostylic type, on the contrary, it becomes very large, and 

 may give rise to a separate ventral element, the symplectic. The 

 amphistylic type appears to be the least specialised of the three, 

 and here the hyomandibular is only moderately developed. The 

 structure of the suspensorial apparatus is of great importance in 

 classification, but it must be borne in mind that convergence may 

 occur. For instance, a pseud-autostylism has been almost certainly 



