The branchial arches are five in number on each side. In the median ventral line, and immediately following the basi-hyal, are three basi-branchial bones. The first arch consists, proceeding from its dorsal to its ventral end, of a small pharyngo-branchial, or superior pharyngeal bone, an epi-branchial, to which is articulated, at a sharp angle, a cerato-branchial followed by a hypo-branchial. The three next arches have a similar composition. The pharyngo-branchials are, however, both large and dentigerous. The hypo-branchial element in the third arch is applied laterally to the last basi-branchial, and is wanting altogether in the fourth arch. The fifth arch consists of a single bone, probably homologous with a cerato-branchial. Osseous plates bearing fine teeth are implanted on the anterior and posterior aspects of the four first arches. These plates are carried by cartilaginous rods in the case of the first cerato-branchial. The rods, known as gill-rakers, are present in some fish on the following cerato-branchials. The interlocking of one series with another, closes the gill-clefts and prevents the passage of foreign bodies of any size.
The vertebral column consists of forty-two vertebrae, of which twenty-two are dorsal and twenty caudal. There are no other regions differentiated in the backbone of a fish. Successive vertebrae articulate one with another by the edges of the centra, which are united by ligaments. In the majority of vertebrae a process springs from the base of the neural arch on each side and articulates with the arch of the vertebra preceding. The neural arches are continuous with the centra, and are prolonged dorsally into neural spines. Lateral projections, or inferior, i. e. haemal arches, as it appears from development, project from the centrum of the eighth and following vertebrae. They increase in length and are turned more and more ventrally in the posterior dorsal vertebrae. The processes of the two last dorsal vertebrae have the ribs united to them, as is the case in the caudal series, where the ventral ends of the ribs unite and inclose the caudal canal, which lodges the caudal artery and vein. This is the case as far as the last or terminal caudal vertebra, which bears solid inferior arches, to either side of which pass the two vessels into which the caudal artery and vein divide. The two last caudal vertebrae are modified.
The last but one has a short expanded neural, and a long inferior arch. The last has the centrum prolonged into the urostyle - the ossified and undivided sheath of the notochord - which is bent upwards dorsally at a sharp angle and incloses the termination of the notochordal cartilage. It has a long, low neural arch and six inferior or haemal arches, expanded and flattened, divided by an interval into two groups of three arches apiece, one anterior, the other posterior. To the neural arches of these two terminal vertebrae are attached three long and somewhat broad 'false' spines, as they are called. These, with the haemal arches, carry the rays of the caudal fin. Simple curved and free ribs are carried on the lateral processes of the dorsal vertebrae except the two last. These ribs bend ventrally but do not meet, and in the Perch, as in all other fish, there is no sternum. Certain of the anterior ribs have epipleural bones attached to them at some little distance from their vertebral ends. These bones radiate outwards into the myocommata or connective tissue septa uniting successive muscular segments or myomeres.
The Perch has four fins belonging to the median or azygos system. These fins, as in all fish, are supported by fin-rays, ossified in the Teleostei, The four fins are the first and second dorsal, the caudal, and an anal, which is in position ventral. The dorsal and anal fins are supported by a series of bones,'fin-bearers' or 'interspinal' bones, which in the case of the dorsal fins alternate with the neural arches; in the case of the anal fin with the inferior arches. These bones lie between the muscle-masses of the right and left halves of the body respectively. The folds of integument which constitute the fins proper, are supported by bony 'fin-rays.' There are thirteen (or sometimes fifteen) of these in the first dorsal, and as in all Acanthopteri with two dorsal fins, they are stout stiff spines, hence called entire fin-rays. The first fin-ray of the second dorsal, and the two first of the anal fin are likewise entire, while the remaining rays as well as all the rays of the caudal fin are 'soft' and 'jointed.' The soft rays break up into filaments at their free termination, and these filaments as well as the stem to a certain extent, are jointed. The caudal fin in the Perch appears externally to be equally developed in its dorsal and ventral half.
It may be termed 'homocercal.' But, anatomically speaking, the eight or nine long fin-rays which make up the bulk of the fin are all articulated to inferior arches, and the upward-bent termination of the notochord has therefore in reality the chief part of the fin on its ventral edge. The dozen or so small 'accessory' rays which belong to the dorsal aspect are inconsiderable in size; and, balanced by similar rays on the ventral aspect, complete the wedge-shaped outline of the fin. The fin is, therefore, from an anatomical point of view, as 'heterocercal' as is the caudal fin of an Elasmobranch in outward appearance as well as anatomically.
The paired fins are the pectoral and pelvic or ventral, i.e. the fore and hind-limbs respectively. A forked bone, the supra-temporal scale, connects the fore-limb to the skull. It is followed by two thin, plate-like bones, developed in membrane and properly belonging to the skin, the so-called supra-clavicle and clavicle. These two bones lie immediately below the mucous membrane of the hind wall of the branchial cavity. They are probably not represented in Vertebrata above the class Pisces. The two 'clavicles' meet in the median line where they are united by ligament. They bear on their inner and posterior face the true shoulder-girdle. This consists of two flat bones, one, the scapula, more dorsal, small, and perforated by a foramen, the other, the coracoid, large and extending nearly to the median line. There is a thin bone, the 'post-clavicle,' which is attached proximally to the clavicle, and hides the scapula when viewed from within. It is superficial and underlies the surface of the depression internal to the pectoral limb. It has been removed on the right side. To the edge of both scapula and coracoid articulates the fore-limb. The small basal cartilages which immediately articulate with the shoulder-girdle cannot be made out in this specimen.