The Vertebrate Skeleton

by Sidney H. Reynolds

There are two principal types of suspensorium by means of which the jaws are attached to the cranium:—

(1) The Autostylic. This is the primitive condition in which the mandibular arch articulates with the base of the cranium in front of the hyoid and in a similar manner.

(2) The Hyostylic. In this case the mandibular arch becomes connected with the hyomandibular and supported by the hyoid arch. These terms are more fully discussed in Chapter VIII.

There is always an internal framework supporting the gills; it usually consists of the hyoid arch and five, rarely six or seven, pairs of branchial arches. The limbs are represented by two pairs of fins, the pectoral and the pelvic; they are not divided into proximal, middle and distal portions. The ribs do not unite with a median ventral sternum, or meet in the mid-ventral line in any other way in the trunk region.

Order I. Elasmobranchii.

The exoskeleton is in the form of placoid scales which are sometimes so numerous as to give the whole skin a rough surface forming shagreen. In some cases the placoid scales are enlarged to form plates or spines capped or coated with enamel. These spines may be imbedded in the flesh in front of the paired or unpaired fins, or may be attached to the tail. They are specially characteristic of the suborder Acanthodii. The endoskeleton is cartilaginous and true bone is never found. Much of the skeleton, especially of the vertebral column, is however often calcified, this being especially well seen in the anterior part of the vertebral column of Rays (Raiidae). In living forms cartilaginous biconcave vertebrae are always well developed, but in some extinct forms the notochord persists unconstricted. Neural and haemal arches are however always developed; they sometimes remain separate, sometimes fuse with the centra. Ribs are often wanting and when present are often not separated off from the vertebrae. The cranium is a simple cartilaginous box whose most prominent parts are the capsules which enclose the sense organs. The skull is sometimes immovably fixed to the vertebral column, sometimes articulates with it by means of two condyles. There is no operculum and no representative of the maxilla or premaxillae. The teeth are very variable. Large pectoral and pelvic fins always occur.

The Elasmobranchii may be divided into four suborders:—

(1) Ichthyotomi.

(2) Pleuropterygii.

(3) Selachii.