Page:EB1911 - Volume 14.djvu/273

ANATOMY] name have been evolved independently from the ancestral covering of placoid elements.

Teleostei.—It will be convenient to take as the basis of our description the bony skeleton of such a Teleostean fish as the salmon. In the vertebral column all the cartilaginous elements are replaced by bone. The haemal spines of the turned-up tip of the tail are flattened (hypural bones) and serve to support the caudal fin rays.

In Argyropelecus and in one or two deep-sea forms the vertebral column remains cartilaginous.

Apart from the ossification of the radialia which takes place in the adults of bony fishes there exist special supporting structures in the fins (paired as well as median) of all the gnathostomatous fishes and apparently in nature independent of the cartilaginous skeleton. These are known as dermal fin-rays. Morphologically they are probably to be looked on (like placoid elements) as local exaggerations of the basement membrane.

In their detailed characters two main types of dermal fin-ray may be recognized. The first of these are horny unjointed rays and occur in the fins of Selachians and at the edge of the fins of Teleostomes (well seen in the small posterior dorsal or “adipose” fin, particularly in Siluroids). The second type of dermal fin-ray is originally arranged in pairs and forms the main supports of the fin in the adult Teleost (fig. 23). The members of each pair are in close contact except proximally where they separate and embrace the tip of one of the radialia. The fin-rays of this second type are frequently branched and jointed: in other cases they form unbranched rigid spines.

In the angler or fishing-frog (Lophius) the anterior rays of the dorsal fin become greatly elongated to form small fishing-rods, from which depend bait-like lures for the attraction of its prey.

In the skull of the adult salmon it is seen that certain parts of the chondrocranium (fig. 24) have been replaced by bone (“cartilage bones”) while other more superficially placed bones (“membrane bones”) cover its surface (fig. 25). Of cartilage bones four are developed round the foramen magnum—the basioccipital, supraoccipital and two exoccipitals. In front of the basioccipital is the basisphenoid with an alisphenoid on each side. The region (presphenoidal) immediately in front of the basisphenoid is unossified, but on each side of it an orbitosphenoid is developed, the two orbitosphenoids being closely approximated in the mesial plane and to a certain extent fused, forming the upper part of the interorbital septum. In the anterior or ethmoidal portion of the cranium the only cartilage bones are a

pair of lateral ethmoids lying at the anterior boundary of the orbit. A series of five distinct elements are ossified in the wall of the auditory or otic capsule, the prootic and opisthotic more ventrally, and the sphenotic, pterotic and epiotic more dorsally. The roof of the cranium is covered in by the following dermal bones—parietals (on each side of the supraoccipital), frontals, dermal ethmoid and small nasals, one over each olfactory organ. The floor of the cranium on its oral aspect is ensheathed by the large parasphenoid and the smaller vomer in front of and overlapping it. The cartilaginous lower jaw is ossified posteriorly to form the articular (fig. 25) with a small membrane bone, the angular, ventral to it, but the main part of the jaw is replaced functionally by a large membrane bone which ensheaths it—the dentary—evolved in all probability by the spreading outwards of bony tissue from the bases of the placoid elements (teeth) which it bears. The original upper jaw (palatopterygoid bar) is replaced by a chain of bones—palatine in front, then pterygoid and mesopterygoid, and posteriorly metapterygoid and quadrate, the latter giving articulation to the articular bone of the lower jaw. These representatives of the palatopterygoid bar no longer form the functional upper jaw. This function is performed by membrane bones which have appeared external to the palatopterygoid bar—the premaxilla and maxilla—which carry teeth—and the small scale-like jugal behind them. The quadrate is suspended from the skull as in the Selachians (hyostylic skull) by the upper portion of the hyoid arch—here represented by two bones—the hyomandibular and symplectic. The ventral portion of the hyoid arch is also represented by a chain of bones (stylohyal, epihyal, ceratohyal, hypohyal and the ventral unpaired basihyal), as is also each of the five branchial arches behind it. In addition to the bony elements belonging to the hyoid arch proper a series of membrane bones support the opercular flap. Ventrally there project backwards from the ceratohyal a series of ten overlapping branchiostegal rays, while more dorsally are the broader interopercular, subopercular and opercular.

In addition to the bones already enumerated there is present a ring of circumorbital bones, a preopercular, behind and external to the hyomandibular and quadrate, and squamosal, external to the hinder end of the auditory capsule.