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  Section: Anatomy of Vertebrate Animals » The Classification and Organization of the Mammalia
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The Ornithodelphia


Ornithodelphia are those Mammals which approach nearer to the Sauropsida, although separated from them by all the essential characters of the Mammalia which have already been defined.

The two genera Echidna and Ornithorhynchus, which constitute this division, agree with one another, and differ from all other Mammals, in the combination of the following characters:

In the spinal column, the centra of the vertebrae are devoid of epiphyses. The os odontoideum, or so-called "odontoid process" of the second cervical vertebra remains for a long time, if not throughout life, unanchylosed with the body of that vertebra, as is the case in many Reptiles. And some of the cervical ribs, in like manner, long persist in a separate condition.

A striking Sauropsidan and Amphibian feature, peculiar to the Ornithodelphia, is seen in the fact that the coracoid, which is a large bone, articulates with the sternum directly. In front of it is another considerable ossification called the epicoracoid, which corresponds in position, though not in the manner of its ossification, with the ossified cartilage so termed in Reptiles. In these Mammals alone, again, there is a T-shaped interclavicle, which supports the clavicles. The central portion of the acetabulum remains unossified, and hence, in the dry skeleton, appears perforated, as in Aves, Ornithoscelida, and Crocodilia.

The inner tendons of the external oblique muscles are ossified for a considerable distance; and these ossifications appear in the dry skeleton as bones, which are articulated with the inner portions of the anterior margins of each pubis. These bones correspond with those which exist in a like position in the Didelphia, and are called marsupial bones; though the term is peculiarly inappropriate, inasmuch as they have nothing to do with the marsupium, or pouch, in which the young are sheltered in most of the Didelphia.

In the upper view of the brain the cerebellum is left completely uncovered by the cerebral hemispheres. The latter are connected by only a very small corpus callosum. The anterior commissure, unlike that of any of the Sauropsida, attains a very great size, and the hippocampal sulcus is prolonged forward to the corpus callosum.

In the internal ear, the cochlea is only slightly bent upon itself, not coiled into a spiral, as in other Mammalia. The stapes is imperforate and columelliform, and the malleus is very large, while the incus is singularly small.

There is a spacious cloaca common to the rectum, genital and urinary organs, as in the Sauropsida and many Ichthyopsida. In both sexes a long urogenital canal opens into the front part of the cloaca. At its anterior end there are five distinct apertures-one in the middle line for the bladder, and two on each side, which are the openings of the genital ducts and of the ureters. Thus in these Mammalia, and in these only, the ureters do not open into the urinary bladder. The testes remain in the abdomen throughout life. The penis is attached to the front wall of the cloaca, and is not united directly with the ischia. It is traversed by a urethral canal, which opens into the cloaca posteriorly, but is not directly connected with either the seminal or the urinary passages. It is probable that, during copulation, the posterior aperture of the penial urethra is applied to the anterior aperture of the urogenital canal, so as to form a continuous passage for the semen.

The ova of the female are very large and project from the surface of the ovary, as in the Sauropsida. The mouths of the Fallopian tubes are not fimbriated. There is no vagina distinct from the urogenital chamber. The mammary glands are situated, one upon each side of the middle line, in the hinder part of the abdominal wall. The various ducts of the gland open upon a small area of the integument which is not raised up into a teat, so that, in the strict etymological sense of the word, these animals are not Mammalia. The mammary gland is compressed by the panniculus carnosus, and not by any prolongation of the cremaster.

There is no sufficient evidence of the nature of the foetal appendages; but the embryo is born in an imperfect condition, and may be provided with a knob or caruncle upon the premaxillae, such as is found in the Sauropsida. In the adult the heart exhibits a fossa ovalis.

Both genera of the Ornithodelphia are restricted to Australia, including Tasmania under that name.

The one of them, Echidna, has the body covered with spines, like a porcupine. It possesses strong digging feet, and a narrow, toothless mouth, from which the long tongue, with which it licks up the ants upon which it preys, is protruded.

The other genus, Ornithorhynchus, has soft fur; a flattened muzzle resembling the beak of a duck, and covered with a leathery integument; and clawed, but strongly webbed feet, fitting it for its altogether aquatic mode of life. The Ornithorhynchus, in fact, frequents fresh-water pools and rivers, very much like a water-rat, sleeping and breeding in burrows excavated in the bank.

In these animals the angle of the mandible is not inflected. They are devoid of any external ear; and, in the males, a kind of spur, which is perforated, and gives exit to the secretion of a gland, is attached to the astragalus. The function of this organ is unknown. In each genus the heart is provided with two superior cavae. In Echidna the right auriculo-ventricle valve is membranous, but, in Ornithorhynchus, it is more or less fleshy.

The hemispheres of the brain are abundantly convoluted in Echidna, but are smooth in Ornithorhynchus. The ovaries are of equal size in Echidna; but, in Ornithorhynchus, the right is much smaller than the left, as in Birds. As has already been stated. Echidna is entirely devoid of teeth, while Ornithorhynchus has four large horny teeth.


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