The Cambridge Natural History Part 12

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CHAPTER VIII

EDENTATA--GANODONTA

ORDER II. EDENTATA

Terrestrial, partly subterranean, or arboreal creatures of quite small to gigantic size (some extinct genera), with frequently a covering of scales or bony scutes. Limbs clawed. Teeth either totally absent or, if present, imperfect in structure, being without enamel, and not forming a complete series; incisors and canines being as a rule absent. Teats axillary, pectoral, or inguinal.[99] Retia mirabilia very common in the extremities.

To this group the name of Bruta was given by Linnaeus, but then it included not only the families which we now place in the modern order Edentata, but also the Elephant and the genus _Trichechus_. Mr. Thomas has proposed to change the name into Paratheria, which name is suggestive of what he and some others think concerning the systematic position of the group, _i.e._ that it is not to be placed in the Eutherian group of mammals at all, but represents a separate twig which has arisen with the Eutheria from a low mammalian stock. This view can hardly be accepted if the Ganodonta--which will be treated of presently--be really ancestral Edentates, for they are not in any way a Prototherian mammalian group, so far as their remains enable us to judge.

The Edentata contain the Sloths, Ant-bears, Armadillos, _Manis_ and _Orycteropus_, among living forms. The great Ground-Sloths, _Megatherium_, etc., and Armadillos, _Glyptodon_, etc., represent the extinct forms.

The name that has been applied to this group is inappropriate {162} inasmuch as many Edentates have teeth. It is, however, by a number of small tooth-characters that the order can be defined. Thus if teeth are present they are simple in structure, without enamel in the adult condition, though a rudimentary enamel-organ has been discovered in an Armadillo. The teeth, moreover, are not found in the anterior part of the mouth, and they grow from persistent pulps; neither is there much differentiation among them. It is not possible, however, to speak of the Edentates as quite h.o.m.odont, since in _Orycteropus_ there are large cheek-teeth; but there is at any rate not a marked heterodonty in that or in any other Edentate. It used to be said that the Edentates were monophyodont. But the Armadillo _Tatusia_ was subsequently found to possess a second suppressed dent.i.tion, and after this discovery Mr. Thomas proved that _Orycteropus_ is also diphyodont.

Since then other Armadillos have been shown to be diphyodont; and the whole group therefore, so far as concerns those members that have teeth, may in all probability be regarded as typically mammalian in this respect.

These characters are slender enough, but there seem to be no others by means of which the members of this order can be satisfactorily linked together. The fact is, that we have here a polymorphic order which contains in all probability representatives of at least two separate orders. We have at present a very few, and these perhaps highly modified, descendants of a large and diverse group of mammals. For convenience' sake they will be all treated of under the head of Edentata.

Although for the probable reasons already stated it is a hard matter to frame such a definition as will include all existing Edentates, it is easy enough to define two groups in this heterogeneous order; to define one group we should say, rather, and then to regard the leavings as forming another not so easily definable a group.

The perfectly-definable group is that which includes the American Anteaters, the Armadillos, and the Sloths. In all these creatures, which may certainly be regarded as representing on their own account as many family types, there are a number of important and highly-characteristic anatomical features which they share in common. So exceedingly different are these three types in general appearance and (correlated with that) in way of life that these common characters acquire increased importance.

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[Ill.u.s.tration]

FIG. 89.--Great Anteater (_Myrmecophaga jubata_). A, Side view of twelfth and thirteenth thoracic vertebrae. B, Posterior surface of second lumbar vertebra. C, Anterior surface of third lumbar vertebra, 2/3. _az_, Anterior zygapophysis; _az_^1, _az_^2, _az_^3, additional anterior, articular facets; _cc_, facet for capitulum of rib; _m_, metapophysis; _pz_, posterior zygapophysis; _pz_^1, _pz_^2, _pz_^3, additional posterior articular facets; _t_, transverse process; _tc_, facet for articulation of tubercle of rib. (From Flower's _Osteology_.)

The first of these characters is the series of additional zygapophyses on the posterior dorsal and lumbar vertebrae; these are very clear in the Anteaters and Armadillos; less clear, but still obviously represented, in the Sloths. In the second place, they all possess a clavicle, rudimentary, it is true, in the Great Ant-bear, but still present. Thirdly, the testes are abdominal throughout life, a character which they share with such lowly-organised animals as the Monotremata and the Whales. Finally, and this is by no means a matter to be overlooked, not only are all the existing members of this group American in range, but there is no evidence to prove that they have ever existed elsewhere. No European or Old-World {164} representatives have as yet been discovered which can be referred to the Anteater, Armadillo, or Sloth type with certainty.[100]

Of these American forms, which will be treated of first, the Armadillos are further apart from either Sloths or Anteaters than the last two are from each other. The name XENARTHRA has been suggested for the American Edentates with "abnormal" vertebral articulations; the corresponding NOMARTHRA includes the Old-World forms.

[Ill.u.s.tration]

FIG. 90.--Right scapula and clavicle of Two-toed Sloth (_Choloepus hoffmanni_). 1-2/3. _a_, Acromion; _af_, prescapular fossa; _c_, coracoid; _cl_, clavicle; _csf_, coraco-scapular foramen; _gc_, glenoid cavity; _pf_, postscapular fossa. (From Flower's _Osteology_.)

Between the Sloths and Anteaters the extinct _Megatherium_ and some of its allies are to a certain extent intermediate. But it may be pointed out in the first place that there are certain important resemblances between the living forms. In both, retia mirabilia are developed in the tail (in spite of its reduction in the Sloths) and in the limbs. But, as is well known, retia are also found in other mammals far removed in the series from these under consideration. The reproductive organs generally are very similar, and they have both a dome-shaped and deciduate placenta. The latter character they share with the Armadillos and with the Aard Vark; _Manis_ having a non-deciduate placenta which is, like that of the Carnivora, zonary in form. The Edentates, at any rate the American forms, have a double vena cava posterior and no azygos vein. This condition is also met with among Whales.

Osteologically the Sloths and Anteaters are united by the fact that the coracoid becomes fused with the coracoid border of the scapula, thus forming a foramen; the importance of this character is, however, discounted by its occurrence in three genera of Cebidae.

The above facts embody the views of Sir William Flower.[101]

{165} A subsequent study of the brain and of the muscles of these animals has led to results not entirely in harmony with these views.

Dr. Elliot Smith is of opinion,[102] after an exhaustive study of the Edentate brain, that in this region of the body the present group shows very decided points of likeness to the Carnivora; that is, so far as concerns the Anteaters. On the other hand, _Orycteropus_ is as distinctly comparable with a primitive Ungulate type, such as is exemplified by _Moschus_. "If the brain of _Orycteropus_," he remarks, "were given to an anatomist acquainted with all the other variations of the mammalian type of brain, there is probably only one feature which would lead him to hesitate in describing it as an exceedingly simple Ungulate brain. That one feature is the high degree of macrosmatism.[103] _Manis_, on the other hand, does not come especially near to _Orycteropus_. The brain of _Manis_ conforms to a simple type of architecture, which agrees in many points with both those of _Orycteropus_ and the American Edentates; there is not sufficient evidence to show which type it really favours." Elliot Smith would, in fact, agree with Max Weber that it is better, if a division is to be made, to divide the group into three orders:--the Xenarthra (Sloths, Anteaters, and Armadillos), Tubulidentata (_Orycteropus_), and Squamata (_Manis_), instead of into Xenarthra and Nomarthra.

Messrs. Windle and Parsons[104] are disposed to see in muscular similarities reasons for uniting _Manis_ with the American Edentates, though they confess to being unable to place _Orycteropus_; in this animal, they say, "we are more struck by the generalised mammalian arrangement of its muscles than by any special Edentate characters. There are, however, two muscles in _Orycteropus_ which show peculiarities not found elsewhere than in the Edentates";--the triceps, which has more than one scapular head, and the tibialis posticus, which is double. They conclude that _Orycteropus_ "presents some feeble claims to be taken into the order."

We shall here adopt the following divisions. {166}

SUB-ORDER 1. XENARTHRA.

FAM. 1. MYRMECOPHAGIDAE.--The family Myrmecophagidae contains three genera, all South American in range. These genera, _Myrmecophaga_, _Tamandua_, and _Cycloturus_, agree greatly in their outward form. They are all without teeth, and have long snouts and long protrusible tongues. The fur is thick, and they have powerful claws wherewith to break down the strong ant-hills upon whose inhabitants they feed. _Tamandua_ and _Cycloturus_ are arboreal, _Myrmecophaga_ is terrestrial in habit. The claws of the arboreal forms are useful to destroy the bark, and thus bring to light insects which lurk in such situations.

[Ill.u.s.tration]

FIG. 91.--Great Anteater. _Myrmecophaga jubata._ 1/10.

The genus _Myrmecophaga_ contains but one species, the Great Anteater, _Myrmecophaga jubata_. It is a large and handsome animal, with long, s.h.a.ggy, greyish-black hair and a broad white stripe across the shoulder.

The coloration is similar in the two s.e.xes. Including the long and bushy tail it reaches a length of over 7 feet. It is on account of its long tongue and greatly developed salivary glands that this and the allied genera were originally placed with _Manis_. It is the submaxillary glands which are so enormous; they extend back over the chest, and open by three distinct ducts, of which two unite just before the external orifice. {167} Along their course these ducts are provided with a sphincter muscle, which squeezes the secretion towards the external orifice into the mouth-cavity.

The stomach is somewhat gizzard-like. The intestine has no caec.u.m.[105]

The Anteater's great claws are not only serviceable in tearing up the ground to get at its food; armed with them he does not fear, as Mr.

Waterton remarked, "the fatal pressure of the serpent's fold or the teeth of the famished jaguar." An Anteater, too, is more than a match for a big dog, and will rip open its belly with the claws while the dog is vainly trying to make an impression with its teeth upon the s.h.a.ggy hair.

_Tamandua_ is a smaller animal than _Myrmecophaga_, and, as has been stated, is arboreal; a.s.sociated with this habit is a prehensile tail. Like the last genus, _Tamandua_ has a rudimentary clavicle, this bone being well developed in the little _Cycloturus_.

[Ill.u.s.tration]

FIG. 92.--Skull of Anteater (_Myrmecophaga_). Lateral view, _al.sph_, Alisphenoid; _cond_, condyle of mandible; _cor_, coronoid process of mandible; _ex.oc_, exoccipital; _ext.aud_, external auditory meatus; _fr_, frontal; _ju_, jugal; _lcr_, lachrymal; _max_, maxilla; _nas_, nasal; _occ.cond_, occipital condyle; _pal_, palatine; _par_, parietal; _p.max_, premaxilla; _s.oc_, supraoccipital; _sq_, squamosal; _ty_, tympanic. (From Parker and Haswell's _Zoology_).

The skull of the Anteater[106] is very long and low; the fore-part is tubular, and there appear to be no traces of teeth. The premaxilla is very small; the zygomatic arch is imperfect, and does not reach the squamosal behind. A curious feature of this genus, which it shares with some Dolphins and other Whales, is that the pterygoid bones develop palatine plates which meet each other in the middle line, and thus shift the opening of the {168} posterior nares backwards. This is also, of course, a character of various lower vertebrates. Another Whale-like character in the skull is the weak character of the mandible, which does not give off a marked coronoid process. But then in neither group is there much mastication. The tympanic, periotic and squamosal are ankylosed together. A peculiarity of the cervical vertebrae is that (as in the Camels) the vertebrarterial ca.n.a.l of several of the vertebrae perforates the pedicle obliquely. There are fifteen or sixteen dorsal and three or two lumbar vertebrae. The additional zygapophyses upon the former have been already referred to. The mode of articulation of the ribs is highly singular.

[Ill.u.s.tration]

FIG. 93.--Skull of Anteater (_Myrmecophaga_). Ventral view. Letters as in Fig. 92. In addition, _b.oc_, basioccipital; _glen_, glenoid surface for mandible; _pter_, pterygoid. (From Parker and Haswell's _Zoology_.)

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FIG. 94.--Side view of three mesosternal segments of a young Anteater (_Tamandua_), showing the mode of articulation of the sternal rib (_sr_).

_mst_, The upper or inner surface of the mesosternal segment; _sy_, the synovial articulation between the segments. (From Flower's _Osteology_, after Parker.)

Each segment of the sternum (of which there are eight) is separated from the next by a synovial membrane: and it has on either side two facets for articulation with the ribs. The way in {169} which these latter bones are connected with the sternum is curiously like their mode of connexion with the spinal column at their other end. With this may be possibly compared the double articulation of the single rib (which articulates with the sternum) in the Rorquals. In _Cycloturus_ this mode of articulation does not occur.

The ma.n.u.s of _Myrmecophaga_ is five-fingered. Of these the third digit (as in Perissodactyles) is the most prominent; it is at least double the width of the second or third finger; the pollex is very slender. In the little _Cycloturus_ this is carried to a greater extent: the third digit is relatively enormous; the first and the fourth have become quite rudimentary; while the fifth is only just recognisable as a minute ossification.

[Ill.u.s.tration]

FIG. 95.--A, Ma.n.u.s of Great Anteater (_Myrmecophaga jubata_). 1/3. B, Ma.n.u.s of Little Anteater (_Cycloturus didactylus_). 2. _c_, Cuneiform; _l_, lunar; _m_, magnum; _p_, pisiform; _s_, scaphoid; _td_, trapezoid; _tm_, trapezium; _u_, unciform; _I-V_, digits. (From Flower's _Osteology_.)

The chevron-bones in the tail surround a well-developed rete mirabile, a rete being found in precisely the same position in the Eastern _Manis_.

_Tamandua_ has also retia, which are also found in the Spider-monkeys.

_Cycloturus_ is by far the smallest of the Anteaters. It has {170} only two toes on the fore-feet. It is to be distinguished, anatomically, from its larger relatives by the complete clavicle, and by the fact that the pterygoids do not meet in the middle line of the skull. The ribs, too, are unusually wide, as in the Whale _Neobalaena_, and form a bony encas.e.m.e.nt for the body. It has two small caeca. Of fossil Anteaters but little is known. The most interesting form is _Scotaeops_, interesting because it has two small back teeth, which are totally lost in its living allies. The huge Patagonian extinct bird _Phororhacos_, first known by a lower jaw, was at one time regarded as a member of this group on account of the form and edentulous character of the jaw.

[Ill.u.s.tration]

FIG. 96.--Unau, or Two-toed Sloth. _Choloepus didactylus._ 1/5. (After Vogt and Specht.)