With Figures 8 and 9.
The Perch belongs to the order Acanthopteri among Teleostei. It may be taken as an illustration of a highly specialised as opposed to a generalised type of organisation: combining as it does so many of the peculiarities of its own class, rather than showing affinities, as e.g. do the Dipnoi, to other and higher forms of life. Its skeleton is typically Teleostean.
This skeleton is well ossified. The parts of the skull may be most readily identified with the help of Figs. 8 and 9. The occipital region (Fig. 8) is composed of the four typical bones, the supra-occipital (S.O.), which, as usual, is here of very large size; the two ex-occipitals (S.O.), which surround the foramen magnum, each pierced by the glossopharyngeal and vagus nerves (X.) and offering an oblique facet (*) for articulation with the first vertebra; and a basi-occipital (B.O.), which has a cup-shaped posterior surface and articulates with the same vertebra. The basi-sphenoid (B.S.) is small and has the Y-shape typical of Teleostei: the alisphenoid (A.S.) is placed in front and above it. Between the occipital and basi-sphenoidal regions intervenes the auditory capsule, which in Teleostei is very large and ossified from five centres. The bones corresponding to these centres are the epi- -otic (Ep. 0.), the opisth-otic (Op. 0.), to which are attached the upper and lower extremities of a forked bone, the post-temporal scale (not shown in the figure) by which the fore-limb is connected to the skull: the pro-otic (Pr. 0.), a large bone pierced by the fifth and seventh (VII) nerves, and two bones which attain their relative maximum development in the Teleostei, the pter-otic (Pt. O.), and the sphen-otic (Sp. O.),( = post-frontal of Huxley). Between these two bones above, and the pro- and opisth-otic below, are the articular cavities (+) for the two heads of the hyomandibular (H.M. in Fig. 9). As is often the case, the parietal (P.) is a very small bone, and the frontal (F.) a very large one.
The nasal (Na.) is slender. In front of the ali- and basi-sphenoid a cartilage plate, the interorbital septum (I.O.), divides the two orbits. It represents the prae- and orbito-sphenoid regions. Anteriorly to the septum is a large bone, the ecto-ethmoid (E. Et.)( = the pre-frontal of Huxley), the homologue of the lateral masses of the ethmoid in a mammal, and pierced by the single olfactory foramen (I.). The two ecto-ethmoids are separated by a small median mesethmoid. The praemaxilla (P. Mx.)
Fig. 8. - From a specimen (natural size).
Is very large and dentigerous, and of a characteristic shape. The maxilla (Mx.), which is edentulous, lies behind and parallel to it, and does not form the margin of the gape of the mouth. This disposition of the two bones is nearly universal among Teleostei. The base of the cranium is underlaid by two bones - a single vomer (Vo.), dentigerous in front, and a parasphenoid (PS.) which extends back to the basi-occipital. These two bones, as well as the praemaxilla, maxilla, nasal, frontal and parietal are preformed in membrane, the remainder belong to the cartilaginous cranium, which persists as cartilage beneath the parietal and frontal regions.
The jaw apparatus (Fig. 9) is very remarkable. It is connected to the cranium in front by the palatine (Pa.) and behind by the hyomandibular (Hm.). The palatine (Pa.) is dentigerous. There is a slender pterygoid (Pt.) (= ectopterygoid of Huxley), which bears a few teeth anteriorly and descends along the anterior edge of the quadrate, while a thin plate-like meso-pterygoid (M. Pt.) (= entopterygoid of Huxley) extends horizontally inwards. These bones are formed in a bar of cartilage lying in front of the mouth in the embryo. Another bar of cartilage - Meckel's arch - which lies behind the mouth segments transversely. The upper, or proximal portion, forms the metapterygoid (Mt. Pt.) and the quadrate (Qu). The latter has a rounded articular head for the articular element of the lower jaw, and both bones are firmly united to the bones in front and behind them. The lower or distal segment constitutes the lower jaw in part. Its upper end ossifies as the articular (Ar.), from which an angular (An.) is cut off, while the median portion remains cartilaginous and rod-like. It is surrounded by the dentary (D.), a bone formed in membrane, and carrying teeth.
The hyoid cartilage of the embryo gives origin to a pharyngo-hyal element which ossifies as the hyomandibular (Hm.) and symplectic (Sy.) bones. They are connected with the bones of the jaw as well as with some others to be described presently. The hyomandibular has two condyles for articulation with the otic region (f in Fig. 8). It is a large bone and represents, according to Kitchen Parker's most recent researches, the columella auris or stapes of higher forms. The quadrate represents the element of the same name in Amphibians, Reptiles, and Birds, and the incus of Mammals, while the malleus of the last-named group corresponds to the articular of the fish, of Amphibia and Sauropsida. A series of membrane bones well developed in Teleostei and Ganoidei are attached to the posterior edge of the hyomandibular and quadrate bones. These are the prae-operculum (P. Op.), the operculum (Op.), the sub-operculum (S. Op.), and the inter-operculum (I. Op.). They close in laterally the branchial cavity. The remaining portion of the embryonic hyoidan cartilage gives origin to the interhyal or stylo-hyal (S. h.) which articulates between the hyomandibular and symplectic, the epihyal (Ep. h), the cerato-hyal (C. h.), both large flat bones, and two nodular hypo-hyals (H. h.). The arches of the right and left side are united by a median basi-hyals (B. h.) or entoglossal.
A series of membrane bones, the branchio-stegal rays (Bs. r.), are attached to the epi-cerato-hyal. They are formed in the membranous flap, which is a continuation of the opercular apparatus to the hypo-hyal region. A thin median bone, the basi-bran-chiostegal ( = urohyal of Huxley), not shown in the figure, projects backwards from the basi-hyal region towards the ventral ends of the clavicles, so-called, with which it is connected by ligament. It underlies the ventral aorta.
Fig. 9. - From a specimen (natural size).