210 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-branchial 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 hypoblastic epithelium of the pharynx, while the outer por- 
tions of the septa are supported by the branchial rays and the 
extra-branchials, the latter lying directly beneath the external 
skin. The segments of the arches are movably connected with 
one another by hgaments; 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 Plewracanthus, true bone 
is never present. In many Fishes, and notably in the Teleostomi, 
1 Huxley, P.Z.S. 1876, p. 40. 
