The Vertebrate Skeleton. Sidney H. Reynolds

deeply biconcave.

      In some cases many of the anterior vertebrae are ankylosed together and to the skull. The vertebrae often articulate with one another by means of obliquely placed flattened surfaces, the zygapophyses. The centrum in early stages of development is partially cartilaginous, but the neural arches and spines in the trunk at any rate, pass directly from the membranous to the osseous condition.

      Fins.

      The most primitive fins are undoubtedly the unpaired ones, which probably originally arose as ridges or folds of skin along the mid-dorsal line of the body, and passed thence round the posterior end on to the ventral surface, partially corresponding in position and function to the keel of a ship.

      In long 'fish' which pass through the water with an undulating motion such simple continuous fins may be the only ones found, as in Myxine. To support these median fins skeletal structures came to be developed; these show two very distinct forms, viz. cartilaginous endoskeletal pieces, the radiale, and horny exoskeletal fibres, the fin-rays. Mechanical reasons caused the fin to become concentrated at certain points and reduced at intervening regions. Thus a terminal caudal fin arose and became the chief organ of propulsion, and the dorsal and ventral fins became specialised to act as balancing organs.

      In some of the earlier Elasmobranchs, the Pleuracanthidae, the endoskeletal cartilaginous radiale are directly continuous with outgrowths from the dorsal and ventral arches of the vertebrae, and form the main part of the fin. In later types of Elasmobranchs the horny exoskeletal fin-rays have comparatively greater prominence. In bony fish, as has been already stated, the horny fibres are replaced by bony rays of dermal origin, and at the same time complete reduction and disappearance of the cartilaginous radiale takes place.

      The Caudal fin.

      The caudal region of the spinal column in fishes is of special importance. It is distinctly marked off from the rest of the spinal column by the fact that the ventral or haemal arches meet one another and are commonly prolonged into spines, while in the trunk region they do not meet but commonly diverge from one another.

      In some fish the terminal part of the caudal region of the spinal column retains the same direction as the rest of the spinal column. The blade of the caudal fin is then divided into two nearly equal portions, and is said to be diphycercal. This condition is generally regarded as the most primitive one; it occurs in the Ichthyotomi, Holocephali, all living Dipnoi, Polypterus and some extinct Crossopterygii, and a few Selachii and Teleostei. It occurs also in deep-sea fish belonging to almost every group, and under these conditions obviously cannot be regarded as primitive, but must be looked on as a feature induced by the peculiar conditions of life.

      In the great majority of fish the terminal part of the caudal region of the spinal column is bent dorsalwards, and the part of the blade of the caudal fin which arises on the dorsal surface is much smaller than is that arising on the ventral surface. Such a fin is said to be heterocercal.

      Strictly speaking all fish whose tails are not diphycercal have heterocercal tails, but the term is commonly applied to two-bladed tails in which the spinal column forms a definite axis running through the dorsal blade, while the ventral blade is enlarged and generally forms the functional part of the tail. Such heterocercal tails are found in nearly all Elasmobranchii, together with the living cartilaginous Ganoidei, and many extinct forms belonging to the same order; Lepidosteus, Amia, and the Dipteridae among Dipnoi, have tails which, though obviously heterocercal, are not two-bladed.

      The vast majority of the Teleostei and some extinct Ganoidei have heterocercal tails of the modified type to which the term homocercal is applied. The hypural bones which support the lower half of the tail fin become much enlarged, and frequently unite to form a wedge-shaped bone which becomes ankylosed to the last ossified vertebral centrum. The fin-rays then become arranged in such a way as to produce a secondary appearance of symmetry. Some homocercal fish such as the Perch have the end of the notochord protected by a calcified or completely ossified sheath, the urostyle, to which several neural and haemal arches may be attached, and which becomes united with the centrum of the last vertebra; in others such as the Salmon the end of the notochord is protected only by laterally placed bony plates.

      The Skull.

      It is often impossible to draw a hard and fast line between the cranium and the vertebral column. This is the case for instance in Acipenser (fig. 18, 16) among Chondrostei, in Amia among Holostei, and in Ceratodus and Protopterus among Dipnoi. The occipital region of the skull in Amia is clearly formed of three cervical vertebrae whose centra have become absorbed into the cranium, while the neural arches and spines are still distinguishable.