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262
[ANATOMY
ICHTHYOLOGY


In the salmon, pike, and various other Teleosts, extensive regions of the chondrocranium persist in the adult, while in others (e.g. the cod) the replacement by bone is practically complete. Bony elements may be developed in addition to those noticed in the salmon.-In the sturgeon the chondrocranium is en sheathed by numerous membrane bones, but wrtilage bones are absent. In the Crossopterygians 1 the chondrocranium persists to a great extent in the adult, but portions of it are replaced by cartilage bones-the most interesting being a large sphenethmoid like that of the frog. Numerous membrane bones cover the chondrocranium externally. In the Dipneusti” the chondrocranium is strengthened in the adult by numerous bones. One of the most characteristic IS the great palatopterygoid bone which develops very early by the spreading of ossification backwards from the tooth bases, and whose early development probably accounts for the non-development of the palatopterygoid cartilage.

A ppendicular Skeleton.—The primitive pectoral girdle, which in the Dipneusti is strengthened by a sheath of bone, becomes in the Teleostomes reduced in size (small scapula and coracoid bones) and replaced functionally by a secondary shoulder girdle formed of superficially placed membrane bones (supraclavicular and c lei thrum or “ clavicle, ” with, in addition in certain cases, an infraclavicular and one or two postclavicular elements), and connected at its dorsal end with the skull by a post-temporal bone.

The pelvic girdle is in Teleostomes completely absent as a rule. The skeleton of the free limb undergoes ossincation to a less or greater extent in the Teleostomes.

In Polypterus the pectoral fin (fig. 18, B) shows three ossification's in the basal part of the fin-pro-, meso- and metapterygium. Of these the metapterygium probably represents the ossified skeletal axis: while the propterygium and also the numerous diverging radrals probably represent the lateral rays of one side of the archipterygium. In the Teleostomes the place of the pelvic girdle is taken functionally by an element apparently formed by the fusion of the basal portions of several radials.,

Vascular S ystem.-The main components of the blood vascular system in the lower vertebrates are the following: (1) a single or double dorsal aorta lying between the enteron and notochord; (2) a ventral vessel lying beneath the enteron; and (3) a series of paired hoop-like aortic arches connecting dorsal and ventral vessels round the sides of the pharynx. The bloodstream passes forwards towards the head in the ventral vessel, dorsal wards through the aortic arches, and tail wards in the dorsal aorta.

The dorsal aorta is single throughout the greater part of its extent, but for a greater or less extent at its anterior end (circulus cephalic us) it consists of two paired aortic roots. It is impossible to say whether the paired or the unpaired condition is the more primitive, general morphological conditions being in favour of the latter, while embryological evidence rather supports the former. The dorsal aorta, which receives its highly oxygenated blood from the aortic arches, is the main artery for the distribution of this oxygenated blood. Anteriorly the aortic roots are continued forwards as the dorsal carotid arteries to supply the head region. A series of paired, segment ally-arranged arteries pass from the dorsal aorta to supply the muscular body wall, and the branches which supply the pectoral and pelvic hns (subclavian or brachial artery, and iliac artery) are probably specially enlarged members of this series of segmental vessels. Besides these paired vessels a varying number of unpaired branches pass from dorsal aorta to the wall of the alimentary canal with its glandular diverticula (coeliac, mesenteric, rectal). The ventral vessel undergoes complicated changes and is represented in the adults of existing fishes by a series of important structures. Its post-anal portion comes with the atrophy of the post-anal gut to lie close under the caudal portion of the dorsal aorta and is known as the caudal vein. This assumes a secondar serving to convey thefood-laden blood from the wall of the enteron to the capillary network of the liver-is known as the hepatic portal vein. The short section in front of the liver is known as the hepatic vein and this conveys the blood, which has been treated by the liver, into a section of the ventral vessel, which has become highly muscular and is rhythmically contractile. This enlarged muscular portion, in which the contractilityprobably once common to the main vessels throughout their extent-has become concentrated, serves as a pump and is known as the heart. Finally the precardiac section of the ventral vessel-the ventral aorta-conveys the blood from heart to aortic arches. .

In addition to the vessels mentioned a large paired vein is developed in closing relation to the renal organ which it serves to drain. This is the posterior cardinal. An anterior prolongation (anterior cardinal) serves to drain the blood from the head region. From the point of junction of anterior and posterior cardinal a large transverse vessel leads to the heart (ductus Cuvieri). H earl.-Originally a simple tube curved into a. somewhat S-shape, the heart, by enlargements, constrictions and fusions of its parts, becomes .converted into the complex, compact heart of the adult. In this we recognize the following portions-e (1) Sinus venosus, (2) Atrium, (3) Ventricle. A fourth chamber, the canus arteriosus, the enlarged and contractile hinder end of the ventral aorta, is also physiologically a part of the heart. The sinus venosus receives the blood from the great veins (ductus Cuvieri and hepatic veins).

It-like the atrium which it

enters by an opening guarded

by two lateral valves-has

thin though contractile walls.

The atrium is as a rule single,

but in the Dipnoansfin' correlation

with 'the"importance (of

their pulmonary breathing, it

is incompletely divided into

a right and a left auricle. In

Neoceratodus the incomplete

division is effected by the

presence of a longitudinal

shelf projecting into the atrial

cavity from its posterior wall.

The opening of the sinus

venosus is to the right of this

shell, that of the pulmonary

vein to the left. In Prototerus

and Lepidosiren a nearly complete

septum is formed by

the fusion of trabeculae, there

vit- vw WV b

I i a a ' 'ba ' a

" V y .

a.

5

U VL VL vb

A B C

From Boas, Lchfbuch der Zaolagie, by permission of Gustav Fischer.

FIG. 26.—Diagram to illustrate

the condition of the Conus in an

Elasmobranch (A), Amia (B) and

a typical Teleost (C).

a, Atrium.

b.a, Bulbus aortae.

c.a, Conus arteriosus.

s.v, Sinus venosus.-11,

v', Valves.

v.a, Ventral aorta.

vt, Ventricle.

being only a minute opening in it posteriorly. The atrium opens by a wide opening guarded by two or more flap valves provided with chordae tendineae into the ventricle. The ventricle, in correspondence with it being the main Y .

connexion with, and drains its blood into, the posterior cardinal veins (see below). In the region between cloaca and liver the ventral vessel becomes much branched or even reticular and-R. H. Traquair, ]01A77l. Anal. Phys. v. (187I) 166; ]. S."Budgett, I Trans. Zool. Soc. Land. xvi. I

3 5-

"I'. W. Bridge, Trans. Zool. Soc. Land. xiv. (1898) 350; W. li. Agar, op. vit.

pumping apparatus, has its walls much thickened by the development of muscular trabeculae which, in the lower forms separated by wide spaces in which most of the blood is contained, become in the Teleostomes so enlarged as to give the wall a compact character, the spaces being reduced to small scattered openings on its inner surface. In. the Dipnoans the ventricle, like the atrium, is incompletely divided into a right and left ventricle. In Ceratodus this is effected by an extension of the inter auricular shelf into the ventricle. In Lepidosiren the separation of the two ventricles is complete but for a small perforation anteriorly, the heart in this respect showing a closer approximation to the condition in the higher vertebrates than is found in any Amphibians or in any reptiles except the Crocodilia. The conus arteriosus is of interest from the valvular arrangements in its interior to prevent regurgitation of blood from ventral aorta into ventricle. In their simplest condition, as seen e.g. in an embryonic Selachian, these arrangements consist of three, four or more prominent longitudinal ridges projecting into thelumen

of the conus, and serving to obliterate the lumen when jammed