Page:EB1911 - Volume 06.djvu/256

Rh exceeds i. $2⁄3$, c. $1⁄1$, p. $3⁄3$, m. $3⁄3$; total 38. Despite the forward position of the teats, which is merely an adaptive feature, bats are evidently mammals of low organization, and are most nearly related to the Insectivora.

In consequence of the backward direction of the knee, a bat, when placed on the ground, rests on all fours, having the knees directed upwards, while the foot is rotated forwards and inwards on the ankle. Walking is thus a kind of shuffle; but, notwithstanding a general belief, bats can take wing from the walking posture.

The bones of the skeleton are characterized by their slenderness and the great size of the medullary canals in those of the extremities. The vertebral column is short, and the vertebrae differ but slightly in number and form throughout the group. The general number of dorso-lumbar vertebrae is 17, whereof 12 are dorsal; the cervical vertebrae are broad, but short. Except in fruit-bats (Pteropodidae), the vertebrae, from the third cervical backwards, are devoid of spinous processes. From the first dorsal to the last lumbar the vertebral column forms a single curve, most pronounced in the lumbar region. The bodies of the vertebrae are but slightly movable on each other, and in old individuals become partially welded. The caudal vertebrae are cylindrical bones without processes; their number and length varying in allied species. The development of these vertebrae is correlated with habits, the long tail in the insectivorous species supporting and controlling the position of the interfemoral membrane which aids bats in their doubling motions when in pursuit of insects by acting as a rudder, and assists them in the capture of the larger insects. In the fruit-bats this is not required, and the tail is rudimentary or absent. In all bats the presternum has a prominent keel for the attachment of the great pectoral muscles.

The shape of the skull varies greatly; but post-orbital processes are developed only in some Pteropodidae and a few Nycteridae and Emballonuridae; in Pteropus leucopterus alone does a process from the zygomatic arch meet the post-orbital so as to complete the orbital ring. Zygomatic arches, though slender, are present in all except in some of the species of Phyllostomatidae.

The milk-teeth differ from those of all other mammals in that they are unlike those of the permanent series. They are slender, with pointed recurved cusps, and are soon shed, but exist for a short time with the permanent teeth. In the Rhinolophidae the milk-teeth are absorbed before birth. The permanent teeth exhibit great variety, sometimes even in the same family, as in Phyllostomatidae, whilst in other families, as Rhinolophidae, the resemblance between the dentition of species differing in many respects is remarkable. In all they are provided with well-developed roots, and their crowns are acutely tuberculate, with more or less well-defined W-shaped cusps, in the insectivorous species, or variously hollowed out or longitudinally grooved in the frugivorous kinds.

The shoulder-girdle varies but slightly, the clavicle being long, strong and curved; and the scapula large, oval and triangular, with a long curved coracoid process. The humerus, though long, is scarcely two-thirds the length of the radius; and the rudimentary ulna is welded with the radius. A sesamoid bone exists in the tendon of the triceps muscle. The upper row of the carpus consists of the united scaphoid, lunar and cuneiform bones.

The “hand” has five digits, the first, fourth and fifth of which consist each of a metacarpal and two phalanges; but in the second and third the number of phalanges is different in certain families. The first digit terminates in a claw, most developed in the frugivorous species, in most of which the second digit is also clawed, although in other bats this and the remaining digits are unarmed.

In the weak pelvis the ilia are long and narrow, while in most species the pubes of opposite sides are loosely united in front in males, and widely separated in females; in the Rhinolophidae alone they form a symphysis. Only in the Molossinae is there a well-developed fibula; in the rest this bone is either very slender or cartilaginous and ligamentous in its upper third, or reduced to a small bony process above the heel, or absent. The foot consists of a short tarsus, and of slender, laterally compressed toes, with much-curved claws.

Although the brain is of a low type, probably no animals possess so delicate a sense of touch as Chiroptera. In ordinary bats tactile organs exist, not only in the bristles on the sides of the muzzle, but in the sensitive structures forming the wing-membranes and ears, while in many species leaf-like expansions surrounding the nasal apertures or extending backwards behind them are added. These nose-leaves are made up partly of the extended and thickened integument of the nostrils, and partly of the glandular eminences occupying the sides of the muzzle, in which in other bats the sensitive bristles are implanted.

In no mammals are the ears so developed or so variable in form; in most insectivorous species they are longer than the head, while in the long-eared bat their length nearly equals that of the head and body. The form is characteristic in each of the families; in most the “earlet,” or tragus, is large, in some cases extending nearly to the outer margin of the conch; its office appears to be to intensify and prolong the waves of sound by producing undulations in them. In the Rhinolophidae, the only family of insectivorous bats wanting the tragus, the auditory bullae reach their greatest size, and the nasal appendages their highest development. In frugivorous bats the ear is simple and but slightly variable. In all bats the ears are extremely mobile, each independently at will.

The oesophagus is narrow, especially in blood-sucking vampires. The stomach presents two types of structure, corresponding respectively to the two divisions of the order, Megachiroptera and Microchiroptera; in the former the pyloric extremity is, with one exception, elongated and folded upon itself, in the latter simple; an exceptional type is met with in the blood-suckers, where the cardiac extremity is elongated, forming a long appendage. The intestine is comparatively short, varying from one and a half to four times the length of the head and body; longest in the frugivorous, shortest in the insectivorous species. In Rhinopoma and Megaderma a small caecum has been found. The liver is characterized by the great size of the left lateral lobe, which occasionally equals half that of the whole organ; the right and left lateral fissures are usually very deep; in Megachiroptera the spigelian lobe is, with one exception, ill defined or absent, and the caudate is generally large; but in Microchiroptera the former lobe is large, while the caudate is small. The gall-bladder is generally well developed.

In most species the hyoids are simple, consisting of a chain of slender, long, cylindrical bones connecting the basi-hyoid with the skull, while the pharynx is short, and the larynx shallow with feebly developed vocal cords, and guarded by a short pointed epiglottis. In the African epauletted bats, Epomophorus, the pharynx is long and capacious, the aperture of the larynx far removed from the fauces, and, opposite to it, opens a canal, leading from the nasal chambers, and extending along the back of the pharynx; the laryngeal cavity is spacious and its walls are ossified; the hyoids are unconnected, except by muscle with the skull; while the cerato-hyals and epi-hyals are cartilaginous and expanded, entering into the formation of the walls of the pharynx, and (in males of some species) supporting the orifices of a pair of air-sacs communicating with the pharynx (fig. 2).

The extent and shape of the wings generally depend on the form of the bones of the fore-limbs, and on the presence or absence of the tail. The wings consist of an “antebrachial membrane,” which extends from the point of the shoulder along the humerus and more or less of the fore-arm to the base of the thumb, the metacarpal bone of which is partially or wholly included in it; the “wing-membrane” spread out between the elongated fingers, and extending along the sides of the body to the posterior extremities, generally reaching to the feet; and the “interfemoral membrane,” the most variable of all, which is supported between the extremity of the body, the legs and the calcar (fig. 1). The antebrachial and wing membranes are most developed in species fitted only for aerial locomotion which when at rest hang with the body enveloped in the wings;