2,1 FISHES CHAP., 



the hyomandibular is the effective suspensorium by which the 

 upper and lower jaws are connected with the skull, and all 

 Fishes in which this arrangement exists are said to be hyostylic} 

 Behind the hyoid arch follow five branchial arches. Each of 

 these is segmented into a dorsal or pharyngo-iranchial element, 

 followed by an epi-, a cerato-, and a hypo-branchial piece, but the 

 later element is absent in the fifth arch. The lateral halves of 

 the last three arches are connected ventrally by a large median 

 basi-branchial cartilage, but in the first and second arches by the 

 median apposition of their respective hypo-branchial elements. 

 Like the hyomandibular and cerato-hyal segments of the hyoid 

 arch, the epi- and cerato-branchial elements of all the branchial 

 arches except the fifth are fringed along their outer convex 

 margins by a series of branchial rays, and, in addition, there 

 are three pairs of slender, curved, cartilaginous rods, or extra-, 

 branchials, in relation with the distal extremities of the branchial 

 rays of the second, third, and fourth branchial arches. The 

 function of the branchial arches, and their branchial rays, and 

 extra-branchial cartilages, is to support the inter-branchial septa 

 which separate the gill-clefts and carry the vascular gill lamellae. 

 All the arches lie near the inner margins of the septa, close to 

 the hjT^oblastic epithelium of the pharynx, while the outer por- 

 tions of the septa are supported by the branchial rays and the 

 extra-brauchials, the latter lying directly beneath the external 

 skin. The segments of the arches are movably connected with 

 one another by ligaments ; and by the contraction of the branchial 

 muscles the arches may be separated or approximated so as to 

 enlarge or diminish the size of the intervening clefts. 



The labial cartilages are represented by a pair of slender rods 

 in relation with the outer surfaces of the palato-pterygo-quadrate 

 cartilages, and a similar pair in connexion with the Meckelian 

 cartilages. There is also a pair of small cartilages in relation 

 with the nostrils. It is probable that the rods which constitute 

 the lateral elements of the rostrum belong to the same category. 



In the Cyclostomes and the Elasmobranchs the skull is entirely 

 cartilaginous, although it may often be superficially calcified in 

 Elasmobranchs, and although there may even be definitely and. 

 symmetrically arranged calcified plates in Pleur acanthus, true bone 

 is never present. In many Fishes, and notably in the Teleostomi, 



' nu.xley, P.Z.S. 1876, p. 40. 



