Skull of Boa.

Fig. 335. Skull of Boa.

(1908). The most extraordinary skeleton met with among Reptiles, and, indeed, among the Vertebrata generally, is that of the Chelonia, in which the ribs and sternum are both placed quite at the exterior of the body, so as to form a broad dorsal shield called the carapace, and an equally strong ventral plate named the plastron, between which the limbs and the head can be more or less completely retracted.

(1909). Yet notwithstanding this apparent total inversion of the osseous system in the creatures before us, it is interesting to observe by what slight modifications in the arrangement of the elements of the skeleton such prodigious changes are accomplished. This is well exemplified in the construction of the carapax of the common Tortoise (Emys Europceus.) In this well-known animal (fig. 336) the vertebrae of the neck and of the tail present nothing particularly remarkable in their structure; but, being connected together in the ordinary manner, the neck and caudal region of the spine present their usual flexibility. The dorsal vertebrae, however, are strangely distorted, the elements of the upper arch being disproportionately developed, while the bodies remain almost in a rudimentary condition. The superior spinous processes of these vertebrae are flattened and converted into broad osseous plates, which form a longitudinal series along the centre of the back, and are connected together by sutures resembling those of the human cranium. The ribs are changed into broad flat bones, firmly united by suture to each other and also to the lateral margins of the spinous processes of the vertebrae, so that they all form, as it were, a single broad plate: the heads of the ribs are very feebly developed, and the intervals between them and the bodies of the vertebrae filled up with ligament.

The margin of the shield thus formed by the dorsal ribs is further enlarged by a third set of flat bones, apparently representing the sternal ribs of the Crocodile, fixed by suture around the whole circumference of the carapax, which they assist in completing.

Skeleton of Tortoise.

Fig. 336. Skeleton of Tortoise.

(1910). The plastron, or sternum, is made up of nine pieces, which have been proved by M. Geoffroy St.-Hilaire to be the elements of this portion of the skeleton in the most complete state of development in which they are met with. Of these nine elements, eight are disposed in pairs; but the ninth, which is always placed between the four pieces composing the two anterior pairs, is single, and occupies the mesial line. In birds, we shall afterwards find this element of the sternum performing a very important office.

(1911). The bones of the shoulder and of the hip, in the Tortoise (fig. 336), are absolutely placed within the thorax, and articulated to the sides of the vertebral column. The precise homology of the scapular apparatus has not been as yet decidedly pointed out; there are, however, three branches, probably representing the scapula, the clavicle, and the coracoid bone: but in the construction of the pelvis, the ilium, the ischium, and the pubis are identified with facility.

(1912). The muscular movements of Reptiles are ordinarily slow and languid, a circumstance which no doubt depends upon the impurity of their blood, consequent on the imperfect manner in which the circulating fluid is exposed to the influences of respiration. The muscles of these animals are, however, peculiarly tenacious of life, and preserve their irritability and power of contraction for an astonishing length of time after they have even been separated from the body. The muscles of a Turtle will continue to live for days after the creature has been decapitated; and the heart will still contract, when irritated, even many hours after its removal.

(1913). But perhaps the most interesting phenomenon connected with the muscular system of the Reptilia is the progressive development of entirely different sets of muscles as the metamorphosis goes on by which they are converted from their earliest fish-condition to their mature and perfect state. This series of changes, which doubtless takes place in all the higher Vertebrata, is well exemplified in the tadpole of the Erog or Toad, and the different phases of development are in such creatures easily investigated. At first the tadpole presents the muscular structure of a fish, both in the muscles of the expanded and vertical tail and in those of the branchial apparatus. As growth proceeds, the broad muscles of the abdomen become developed; and ultimately those of the limbs are superadded as those members successively make their appearance, the muscles of the shoulder and pelvic region being first recognizable, and subsequently those of the legs and feet. In the meantime, as the abdominal muscles and those of the extremities become gradually perfected, those peculiar to the fish-state are rapidly removed: the broad tail becomes atrophied and absorbed, diminishing in length nearly at the rate of a line a day; the flaky lateral muscles of the caudal region disappear altogether; and, moreover, the entire muscular apparatus of the branchial and hyoid systems is altered as the character of the respiratory organs becomes changed, in a manner to be explained hereafter, from the aquatic to the aerial condition.

(1914). As Reptiles, for the most part, must from necessity swallow their prey entire, organs of taste would be scarcely more useful to them than to the fishes described in the last chapter; and we are therefore not at all surprised to find the tongue in almost every family appropriated to a totally different use, and not unfrequently converted into an apparatus of prehension, whereby the food is seized and conveyed into the mouth.

(1915). In the Batrachoid Amphibia, for instance, we have a remarkable example of this provision. The Prog and the Toad, notwithstanding their slow and clumsy movements, are destined to feed upon insects, and consequently must be provided with some instrument by which such active prey may be caught. The organ provided for this purpose is the tongue, which, by a slight modification in its structure, becomes changed into a prehensile forceps admirably adapted to such an office. The tongue of the Frog, instead of presenting the usual arrangement, is found to be fixed to the symphysis of the lower jaw, and folded back upon itself, so that its point, which is free and bifid, is lodged in the throat. Thus provided, the Frog is enabled to seize its victim with the greatest ease. No sooner does a fly approach sufficiently near, than this living forceps is rapidly everted; and the insect, being seized by its furcate extremity, is as speedily brought between the jaws of its destroyer. The teeth of the Batrachia very much resemble those of the generality of fishes, being simple points soldered to the surface of the jaws, but not implanted in sockets - sufficient to give a secure hold of their food, but quite unadapted to mastication.