1911 Encyclopædia Britannica/Monotremata
MONOTREMATA (a name referring to the single outlet for all the excretory channels of the body), the lowest subclass of the Mammalia, represented at the present day solely by the platypus and the echidnas. It has been proposed to replace this name, when used as a subclass, by Prototheria; but it is perhaps on the whole preferable to retain it both for the subclass and for the single order by which it is now represented, distinguishing the latter as Monotremata Vera.
Existing monotremes are characterized by the following features. In the first place they differ broadly from all other mammals in being oviparous, or possibly in the case of one family ovoviviparous; and also in the absence of mammae, or teats, the milk-glands opening on the surface of the skin of the breast by means of a number of fine pores. Moreover, the milk-glands themselves are commonly believed to represent sweat-glands and not those of other mammals, although it has been suggested that this distinction may not prove to be valid. In the strict sense of the term monotremes are not, therefore, mammals at all. Another feature in which these creatures differ from all other living mammals is the presence of a pair of coracoid bones, which articulate with the sternum, or breastbone, as well as of paired precoracoids, or epicoracoids, and an unpaired T-shaped interclavicle, the arms of which overlie the clavicles or collar-bones. In all these respects monotremes closely resemble many reptiles. The brain lacks a corpus callosum, or band of nerve-tissue connecting the two hemispheres. Again, the bodies of the vertebrae are for the most part without terminal caps, or epiphyses; and each rib articulates to the vertebral column solely by its head or capitulum, instead of by a capitulum and a tuberculum. More important is the circumstance that the testes, which remain throughout life within the abdominal cavity, do not discharge by means of their ureters into a urinary bladder, but into a urino-genital sinus, which is in close communication with the lower end of the alimentary canal, so that the genital and waste products of the body are discharged by means of a common tube, or cloaca—another reptilian feature, although met with in certain other mammals. As regards other soft parts, the heart has the valve dividing the right auricle and ventricle incomplete and to a great extent fleshy—a feature which may, in some degree, account for the lower temperature of monotremes as compared with higher mammals. The presence of an anterior abdominal vein, or at least its supporting membrane, running right through the abdominal cavity, is another distinctive feature of the group. Of less importance is the presence of a pair of epipubic, or marsupial, bones attached to the front edge of the pelvis. The females have a complete or rudimentary pouch on the abdomen.
In the presence of hair, the relatively high temperature of the blood, the absence of nuclei to the red blood-corpuscles, and the existence of only the left aortic arch, as well as in the absence of a separate quadrate-bone, and the simple structure of the lower jaw, monotremes conform to the ordinary mammalian type. On the other hand the skull of the platypus possesses a peculiar “dumbbell bone,” believed to represent the reptilian prevomer.
The females produce their young from eggs, which are relatively large, and develop in the same manner as those of birds and reptiles, a portion only of the yolk segmenting to form the embryo, while the remainder serves for the nutriment of the latter. In the case of Ornithorhynchus it has been said that two eggs are laid in the chamber at the end of the burrow,[1] but those of the Echidnidae are carried about in the pouch on the abdomen of the female, which becomes enlarged during the time of incubation. In the adult state neither of the living groups of Monotremata have teeth; but this is evidently only a specialized feature, the young platypus having functional teeth. In the latter, three pairs of these teeth are developed in the upper, and three in the lower jaw; but after being for some time in use, they gradually become worn away, and are finally shed. Under and around the teeth are developed the horny plates, or “cornules,” which gradually grow round them and assume their function, the hollows on the surface of the cornules indicating the positions of the teeth. In form these teeth make a distant approximation to the molars of some of the extinct Multituberculata (q.v.).
A peculiarity of the males is the presence in the hind-limb of an additional, flat, curved ossicle on the hinder and tibial side of the plantar aspect of the tarsus, articulating chiefly to the tibia, supporting in the adult a sharp-pointed perforated horny spur, with which is connected the duct of a gland situated beneath the skin of the back of the thigh. (A rudimentary spur is found in the young female Ornithorhynchus, but this disappears when the animal becomes adult.) The stomach is sub-globular and simple; the alimentary canal has no ileo-caecal valve, or marked distinction between large and small intestine, but is furnished with a small, slender vermiform caecum with glandular walls. The liver is divided into the usual number of lobes, and is provided with a gall-bladder.
The trunk-vertebrae are nineteen in number. The transverse processes of the cervical vertebrae are independently developed, and remain suturally connected with the bodies of the vertebrae until the animal is full-grown. Though in this respect monotremes present an approximation to reptiles, they differ in that there is not a gradual transition from these transverse processes of the neck-vertebrae (or cervical ribs, as they may be considered) into the thoracic ribs, for in the seventh vertebra the costal element is much smaller than in the other, indicative of a very marked separation of neck from thorax, not seen in reptiles. The sternal ribs are well ossified, and there are distinct, partly ossified, intermediate ribs. The brain-cavity, unlike that of the lower marsupials or reptiles, is large and hemispherical, flattened below, arched above, and about as broad as long. The cribriform plate of the ethmoid is nearly horizontal. The cranial walls are very thin, and smoothly rounded externally, and the sutures become completely obliterated in adults. The broad occipital region slopes upwards and forwards, and the face is produced into a long depressed beak. The bony palate is prolonged backwards, so that the posterior nares are nearly on a level with the glenoid fossa. The lower jaw, or mandible, is without distinct ascending ramus; the coronoid process and angle being rudimentary, and the two halves loosely connected at the symphysis. The fibula has a broad, flattened process, projecting from its upper extremity above the articulation, like an olecranon.
The first family, Ornithorhynchidae, is represented solely by the duck-billed platypus, or platypus (Ornithorhynchus anatinus), in which the hemispheres of the brain are relatively small and smooth, while the muzzle is expanded to form a spatula-like beak, covered during life with a delicate sensitive skin, which dries in museum-specimens to a horny consistency. Although, as mentioned above, functional teeth are developed in the young, in the adult their function is discharged by “cornules,” or horny structures—elongated, narrow and sharp-edged, along the anterior part of the sides of the mouth, and broad, flat-topped or molariform behind. The legs are short and adapted for swimming; the feet webbed, each with five well-developed toes armed with large claws, and beyond which in the fore-feet the interdigital membrane is extended. Vertebrae: C. 7, D. 17, L. 2, S. 2, Ca. 21. Acetabulum of pelvis not perforated. Tongue not extensile. Mucous membrane of small intestine covered with delicate, close-set transverse folds or ridges. Tail rather short, broad, and depressed. Eyes very small. Fur close and soft.
The platypus, or water-mole, is common to Australia and Tasmania, and entirely aquatic in habits, diving freely, and making its burrow in the river-banks. It feeds on insects, snails, small bivalve molluscs, and worms. In the adult state bivalves form its chief food; and it is believed that the substitution of horny plates for brittle teeth is an adaptation for cracking the shells of these creatures. (See Platypus.)
The second family, Echidnidae, has a wider geographical distribution, including Australia, Tasmania and New Guinea, and is represented by two genera. The hemispheres of the brain are large and convoluted; and the muzzle is produced into a long, tapering, tubular beak, at the end of which the nostrils are situated. The two branches of the lower jaw are slender and rod-like. Opening of mouth small, and placed below the extremity of the beak. No teeth, though the palate and tongue are furnished with spines. Tongue very long, vermiform, slender and protractile. Lining membrane of small intestine villous, but without transverse folds. Feet with long strong claws for scratching and burrowing. The hind-feet with the ends of the toes turned outwards and backwards in the ordinary position of the animal when on the ground. Tail very short. Acetabulum with a large perforation. Calcaneal spur and gland of the male much smaller than in Ornithorhynchus. Fur intermixed with strong, sharp-pointed spines. Terrestrial and fossorial in habits, feeding exclusively on ants.
The typical genus Echidna is represented by the echidna, or porcupine-anteater (E. aculeata), which has a distribution equivalent to that of the family, and includes several local races. It is characterized by the presence of five claws to each foot, the moderately long and straight beak, the tapering tongue, with its spines restricted to the basal portion, and the vertebrae numbering C. 7, D. 16, L. 3, S. 3, Ca. 12. In Proechidna, represented by the larger P. bruijni and P. nigroaculeata, both from New Guinea, on the other hand, terminal phalanges and claws are present only on the three middle toes of each foot, the tongue is somewhat spoon-shaped and carries three rows of spines along its upper surface, and there are 17 dorsal and four lumbar vertebrae. (See Echidna.)
Bruijn’s Echidna (Proechidna bruijni).
At present no light is shed by palaeontology on the past history of the Monotremata Vera. Species of Echidna and Ornithorhynchus have indeed been described from the superficial formations of Australia, but they apparently differ in no structural details from their existing representatives.
Possibly some of the extinct Jurassic mammals with a marsupial or insectivorous type of dentition referred to in the article Marsupialia may be monotremes, but there is no definite evidence that this is the case. On the other hand, there is a possibility that another extinct group of mammals, dating from the Trias and continuing till the Lower Eocene, may belong to the present subclass, of which they form a second order. (See Multituberculata.)
The most important recent information with regard to the Monotremata will be found in Dr R. Semon’s Reise in Australien, in the Denkschrift of the Jena Natural History Society. (R. L.*)