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MONOTREMATA (a name referring to the ...
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See also:MONOTREMATA (a name referring to the single outlet for all the excretory channels of the See also:body) , the lowest subclass of the See also:Mammalia, represented at the See also:present See also:day solely by the See also: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 See also:order by which it is now represented, distinguishing the latter as Monotremata See also:Vera. Existing monotremes are characterized by the following features. In the first See also:place they differ broadly from all other mammals in being oviparous, or possibly in the See also:case of one See also:family ovoviviparous; and also in the See also:absence of mammae, or teats, the See also:milk-glands opening on the See also:surface of the skin of the See also:breast by means of a number of See also: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 See also: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 breast-See also:bone, as well as of paired precoracoids, or epicoracoids, and an unpaired T-shaped interclavicle, the arms of which overlie the clavicles or See also:collar-bones. In all these respects monotremes closely resemble many See also:reptiles. The See also:brain lacks a corpus callosum, or See also:band of See also:nerve-See also:tissue connecting the two hemispheres. Again, the bodies of the vertebrae are for the most See also:part without terminal caps, or epiphyses; and each See also:rib articulates to the vertebral See also:column solely by its See also:head or capitulum, instead of by a capitulum and a tuberculum. More important is the circumstance that the testes, which remain throughout See also:life within the abdominal cavity, do not See also:discharge by means of their ureters into a urinary See also:bladder, but into a urino-genital sinus, which is in See also:close communication with the See also:lower end of the alimentary See also:canal, so that the genital and See also:waste products of the body are discharged by means of a See also:common See also:tube, or See also:cloaca —another reptilian feature, although met with in certain other mammals. As regards other soft parts, the See also:heart has the See also:valve dividing the right See also:auricle and ventricle incomplete and to a See also:great extent fleshy—a feature which may, in some degree, See also: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, See also:running right through the abdominal cavity, is another distinctive feature of the See also:group. Of less importance is the presence of a pair of epipubic, or marsupial, bones attached to the front edge of the See also:pelvis. The See also:females have a See also:complete or rudimentary pouch on the See also:abdomen. In the presence of See also:hair, the relatively high temperature of the See also:blood, the absence of nuclei to the red blood-corpuscles, and the existence of only the See also:left aortic See also:arch, as well as in the absence of a See also:separate quadrate-bone, and the See also:simple structure of the lower See also:jaw, monotremes conform to the See also:ordinary mammalian type. On the other See also:hand the See also:skull of the platypus possesses a See also:peculiar " dumb-See also:bell bone," believed to represent the reptilian prevomer. The females produce their See also: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 See also:form the embryo, while the See also: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; but those of the Echidnidae are carried about in the pouch on the abdomen of the See also:female, which becomes enlarged during the See also:time of See also:incubation. In the adult See also:state neither of the living See also:groups of Monotremata have See also:teeth; but this is evidently only a specialized feature, the young platypus having functional teeth. In the latter, three pairs of these teeth are See also: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 See also:shed. Under and around the teeth are developed the horny plates, or " cornules," which gradually grow See also:round them and assume their See also: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 See also:extinct See also:Multituberculata (q.v.). A peculiarity of the See also:males is the presence in the See also:hind-See also:limb of an additional, See also:flat, curved ossicle on the hinder and tibial See also:side of the plantar aspect of the See also:tarsus, articulating chiefly to the See also:tibia, supporting in the adult a See also:sharp-pointed perforated horny See also: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 See also:animal becomes adult.) The See also:stomach is sub-globular and simple; the alimentary canal has no ileo-caecal valve, or marked distinction between large and small See also:intestine, but is furnished with a small, slender vermiform caecum with glandular walls. The See also:liver is divided into the usual number of lobes, and is provided with a See also:gall-bladder. The See also: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 pre-sent an approximation to reptiles, they differ In that there is not a See also:gradual transition from these transverse processes of the See also:neck-vertebrae (or cervical ribs, as they may be considered) into the thoracic ribs, for in the seventh vertebra the costal See also:element is much smaller than in the other, indicative of a very marked separation of neck from See also: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 See also:long. The cribriform See also:plate of the ethmoid is nearly See also:horizontal. The See also: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 See also:face is produced into a long depressed See also:beak. The bony See also:palate.is See also:pro-longed backwards, so that the posterior See also:nares are nearly on a level with the glenoid fossa. The lower jaw, or mandible, is without distinct ascending See also:ramus; the coronoid See also:process and See also: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 See also:articulation, like an olecranon. The first family, Ornithorhynchidae, is represented solely by the See also:duck-billed platypus, or platypus (Ornithorhynchus anatinus), in which the hemispheres of the brain are relatively small and smooth, while the muzzle is See also: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 stuctures—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 See also:short and adapted for See also: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. q, D. 27, L. 2, S. 2, Ca. 21. See also:Acetabulum of pelvis not perforated. See also: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. See also:Fur close and soft. The platypus, or See also:water-See also:mole, is common to See also:Australia and See also:Tasmania, and entirely aquatic in habits, diving freely, and making its burrow in the See also:river-See also:banks. It feeds on See also:insects, snails, small bivalve molluscs, and See also:worms. In the adult state bivalves form its See also:chief See also:food; and it is believed that the substitution of horny plates for brittle teeth is an See also:adaptation for cracking the shells of these creatures. (See PLATYPUS.) 1 There does not appear to be See also:authentic See also:evidence that the eggs in this genus are actually laid. (See PLATYPUS.) The second family, Echidnidae, has a wider See also:geographical See also:distribution, including Australia, Tasmania and New See also: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 See also: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 mailer than in Ornithorhynchus. Fur intermixed with strong, sharp-pointed spines. Terrestrial and fossorial in habits, feeding exclusively on ants. The typical genus See also:Echidna is represented by the echidna, or See also:porcupine-See also:anteater (E. aculeata), which has a distribution Bruijn's Echidna (Proechidna bruijni). See also:equivalent to that of the family, and includes several See also:local races. It is characterized by the presence of five claws to each See also:foot, the moderately long and straight beak, the tapering tongue, with its spines restricted to the basal portion, and the vertebrae numbering C. q, D. i6, L. 3, S. 3, Ca. r 2. 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 See also:middle toes of each foot, the tongue is somewhat See also:spoon-shaped and carries three rows of spines along its upper surface, and there are rq dorsal and four lumbar vertebrae. (See ECHIDNA.) At present no See also:light is shed by palaeontology on the past See also:history of the Monotremata Vera. See also: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 See also:Jurassic mammals with a marsupial or insectivorous type of dentition referred to in the See also:article See also: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 See also:Eocene, may belong to the present subclass, of which they form a second order. (See ,MULTITUBERCULATA.) The most important See also:recent See also:information with regard to the Monotremata will be found in Dr R. Semon's Reise in Australien, in the Denkschrift of the See also:Jena Natural History Society. (R. Additional information and CommentsThere are no comments yet for this article.
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