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

 
     
 

In this group of Edentata the zygoma sends down no process from its lateral region, although, in some rare cases, the anterior part of the arch has a descending prolongation. The acromion and the coracoid do not become united. The scaphoid and the trapezium remain distinct; and the sole of the hind-foot rests upon the ground by a greater or lesser extent of its whole surface, and not merely by its outer edge.

The insectivorous Edentates are divisible into four groups-a. the Mutica, b. the Squamata, c. the Tubulidentata, and d. the Loricata.

a. The group of the Mutica contains the genera Myrmecophaga and Cyclothurus, the Ant-eaters of South America. The bodies of these animals are covered with hair, and they are provided with very long tails, which are sometimes prehensile. The skull is greatly elongated, and the small premaxillae are but loosely connected with it. The jugal arch is incomplete. In Myrmecophaga, the pterygoids, which are very long, stretch back to behind the level of the tympanic bullae, with the whole inner edges of which they are united either by bone or by membrane; and as, at the same time, they unite in the middle line, the roof of the palate is greatly prolonged, and the posterior nares are bounded below and at the sides by the pterygoid bones. This arrangement is to be found in no other Mammals, except some Cetacea, nor in any other Vertebrata, except the Crocodiles. The mandible is very slender, the ascending ramus, coroncid process, and angle of the jaw, being obsolete. The articular surface of the condyle is flat. The hyoid is placed far back beneath the posterioi cervical vertebrae, and is connected with the skull only by muscles. The thyroid and the cricoid cartilages are ossified. The dorso-lumbar vertebrae are complicated by the presence of accessory articular processes. Well-developed clavicles are present in the climbing Cyclothurus didactylus, but they are incomplete, or absent, in the other species. In the manus, the outer digit, or digits, are devoid of claws, and the weight of the body, when the animal walks, is supported upon its outer edge, which is frequently thick and callous. The pes has five digits, each provided with a strong nail, and the sole rests upon the ground.

The tongue is extraordinarily long and protractile; it is not connected to the hyoid by the ordinary hyo-glossus muscles; but long muscles, which are attached to the sternum (sternoglossi), retract it, while it is protracted by the genioglossi and stylo-hyoidei.

Immense submaxillary glands extend back over the thorax, and cover the tongue with a viscid secretion, when it is thrust into the nests of the ants, upon which the Myrmecophaga preys. The insects, entangled by thousands in this substitute for birdlime, are then dragged back into the mouth of the Ant-eater, and swallowed. The pyloric portion of the stomach is so exceedingly thick and muscular as to be comparable to a gizzard. The brain presents numerous convolutions, and has a large corpus callosura. The anterior commissure is also remarkably large. In the female, the uterus is simple, but has a double os uteri. The placenta is said to be discoidal in form in Myrmecophaga didactyla.

b. The group of the Squamata contains the single genus Manis,species of which are found both in Africa and Southern Asia. In these singular animals, the body is covered with overlapping, horny scales, and they have the power of rolling up like hedgehogs. In walking, the long claws of the forefoot are bent under, so that their dorsal surfaces rest upon the ground, while the weight of the hinder part of the body is thrown upon the flat soles of the hind-feet.

The skull is elongated, the premaxilla is small, and the zygoma usually incomplete. The pterygoids are much elongated and extend backward beyond the bullate tympanic bones, but they do not unite in the middle line. The mandible has no ascending ramus, and its condyle is flat. Air-passages in the walls of the skull place one tympanum in communication with the other and extind into the squamosal bone. There are no clavicles. The "xiphoid" extremity of the sternum is large, and may be produced into two long cornua, as in Lizards. The mouth is toothless.

The large salivary glands extend on to the thorax. The stomach is divided into a thin-walled cardiac sac, lined by a dense epithelium, and a thick muscular pyloric portion. It is always found to contain numerous stones. The placenta appears to be diffuse and non-deciduate.

c. The Tubulidentata are also represented only by a single genus, Orycteropus, which is a native of South Africa. The body is hairy, provided with thoracic and inguinal teats, and the ears are long, not short or rudimentary, as in the preceding genera. In both the fore-and the hind-limbs, the foot rests evenly upon the ground and mainly upon the plantar surfaces of the strong claws. The fore-foot has only four digits, in consequence of the absence of the pollex, while the hind-foot is pentadactyle.

The skull has a complete zygoma and well-developed premaxillae. The lachrymal bone is large, and the lachrymal foramen is situated upon the face. The tympanic bone is annular, and the periotic mass so large, and enters so much into the lateral walls of the skull, as to remind one of its proportions in the Sauropsida. The mandible has an ascending ramus. The clavicules are complete.

The jaws are provided with teeth, the substance of which is traversed by a great number of parallel vertical canals. These teeth are rootless molars, and the greatest number which has been observed is 8.8/6.6, but the small anterior ones fall out, reducing them to 5.6/4.4. The hindermost, and the small anterior ones, are simple cylinders, but the middle teeth present a longitudinal groove on each side.

The submaxillary glands are very large. The stomach is divided into a right and a left portion; the former having very thick and muscular walls. The intestine has a cascum. It is stated that the ductus arteriosus long remains open.

The two uteri open separately into the vagina. The placenta is deciduate and discoidal.

d. In the Loricata, the dorsal region of the body is covered by a carapace, composed of epidermal scales, and of suturally united quadrate, or polygonal, scutes, which are dermal ossifications, so that the whole structure is strictly comparable to the dorsal shield of a crocodile. These are the only Mammals in which such scutes exist. When fully developed, the dorsal armor of one of these animals presents five distinct shields, the edges of which permit of a certain amount of motion between them. One of these covers the head, and is called cephalic; another, nuchal, protects the back of the neck; a third, scapular, covers the shoulders like a great cape; a fourth, usually consisting of a number of tree and movable segments, covers the posterior dorsal and lumbar region, as the thoracoabdominal shield; and the fifth, the pelvic, is attached by its deeper surface to the ilia and ischia, and arches over the rump like a half dome. The tail may further be invested by a series of incomplete bony rings and scattered scales; and scutes are distributed over the limbs. In one genus, Chlamydophorus, the scutes are developed only in the pelvic region.

In the skull the premaxillae are well developed, and the zygoma is complete. The mandibular ramus usually has a well-developed ascending portion and coronoid process. Clavicles are present. The fore-and the hind-feet rest upon the ground evenly, and indeed the hind-limbs are usually plantigrade, or nearly so; but, in the singular genus Tolypeutes, the fore-foot is supported upon the extremities of the long nails. The pollex is always present in the fore-foot, but the fifth digit sometimes becomes rudimentary. There are always five toes in the hind-foot.

In the genus Euphractes, each premaxilla contains a single tooth, which, consequently, is an incisor.

This group contains two divisions, the Dasypodidae and the Glyptodontidae; both are South American, but the former is chiefly composed of living animals, while the latter only contains an extinct genus.

The Dasypodidae are what are commonly known by the name of Armadillos. In this division the thoraco-abdominal shield, when present, as it is in all the genera except Chlamydophorus, consists of, at fewest, three, and, at most, thirteen, transverse movable zones of scutes.

In the skull, the ends of the nasal bones project beyond the level of the premaxillae, so that the nasal aperture looks more or less downward. The premaxillae have a considerable size, and articulate largely with the nasals. The anterior part of the jugal arch offers, at most, a rudimentary downward prolongation. The mandibular symphysis has but a moderate length, and the posterior alveoli of the mandibles do not extend along the inner face of the ascending portion of the ramus of the jaw.

The teeth of the upper and lower jaws alternate, and hence their grinding surfaces wear down into ridges.

The odontoid vertebra is anchylosed with a greater or smaller number of its successors. The cervical vertebrae which follow these have peculiar accessory articular surfaces; and the hinder dorsal and the lumbar vertebrae are also provided with accessory articular facets and processes. A number of the anterior caudal vertebrae are always anchylosed with one another, and with the true sacrals, to form the long sacrum; and the transverse processes of some of these caudal vertebrae abut against the inner surfaces of the ischia, and become anchylosed therewith.

The first rib is broad and flattened, and the anterior piece of the sternum is expanded. The succeeding vertebral ribs arc connected by ossified sternal ribs with the sternum, and these are articulated, not only with the sternum, but with one another.

In the carpus, the cuneiform bone bends round the uniform, and articulates with the fifth metacarpal, when that bone is present. The ungual phalanges of the manus are long and pointed. The femur has a third trochanter, and the four inner metatarsals are much longer than they are broad.

The division of the Glyptodontidae contains the single genus Glyptodon, which is essentially a large armadillo; but it departs, in some respects, not only from all these animals, but from all other Mammalia, and even stands alone among the Vertebrata.

The carapace covers the whole body, but presents no movable thoraco-abdominal zones, inasmuch as it consists of polygonal plates firmly united together, and fringed by a margin of scutes with raised conical surfaces.

The nasal bones are short and broad, and their free ends do not project so far as the premaxillae; whence the anterior nasal aperture looks slightly upward as well as forward. The premaxillae, however, are very small bones, and, if they unite with the nasals at all, do so for a very short distance. The anterior portion of the jugal arch gives off a great downward process. The mandibular symphysis is very long, and the posterior alveoli of the mandible are situated upon the inner face of the very high perpendicular part of the ramus. The teeth are trilobed, two deep grooves excavating their inner and their outer surfaces. And, as the crowns of those of each jaw arc placed opposite each other, they are worn flat.

The last cervical and the anterior dorsal vertebrae are anchylosed together into a single "tri-vertebral" bone which moves by a hinge-joint upon the third dorsal. This and the succeeding dorso-lumbar vertebrae are immovably united, and, for the most part, anchylosed, together. The head of the first rib is engaged in the socket furnished to it by the tri-vertebral bone in such a manner as to be immovable, and the rib is not flat, but rounded and columnar.

In the carpus, the cuneiform bone articulates with the fourth, as well as with the fifth metacarpal, the latter bone being entirely supported by the cuneiform. The metacarpals and phalanges are all very short and broad. The pollex is rudimentary, while the fifth digit is fully developed.

The supra-condyloid ridge of the femur is not distinct from the third trochanter, even if the latter can be said to exist at all. The metatarsal bones are as broad as they are long, or broader; and, as in the fore-foot, the majority of the phalangea are comparatively short and truncated.

 
     
 
 
     



     
 
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