Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/812

Rh 768 AMPHIBIA tribution of the hypoglossal; in Menobranchus, however, the corresponding nervous supply is furnished by the second and third cervical nerves, the first spinal nerve, in this genus, perforating the sides of the body of the atlas, and being distributed to a muscle which passes from this vertebra to the occiput (Fischer, I.e., p. 158). In Pipa the hypoglossal is furnished by the second cervical nerve ; in Salamandra, by the first and second. There is no trace of any suboccipital nerve in the Amphibia ; and as, in the absence of this nerve, the first spinal would appear to answer to the second cervical of the higher Verte brate^ the fact that it takes the place of the hypoglossal becomes very perplexing. In the Anura (JKana) the sympathetic is represented by a double chain of ganglia, situated at the sides of the aorta, and receiving branches from the anterior divisions of the spinal nerves. It appears to be continued in the skull by commissural cords which pass forwards on the inner side of tho auditory capsule, and connect the ganglion of the vagua with that of the trigeminal. The Organs of tJte HigJier Senses. The nasal sacs are elongated in Proteus, Menobranchus, and Siren, and not covered by nasal bones or alinasal cartilages. In the other Amphibia they are broader, and enclosed by cranial carti lages and ossifications. The olfactory mucous membrane is variously folded; and, in Rana, some of these folds are supported by ingrowths of the anterior cartilaginous wall of the nasal chamber. In Proteus the eye is completely hidden by the continua tion of the unaltered integument over it, and the organ of vision is almost as much obscured in the Peromela. In the other perennibranchiate Urodela, and in Pipa, the in tegument covering the eye forms a transparent cornea, but there are no eyelids. The abranchiate Urodela have an upper and a lower lid; and, in the higher Anura, the lower lid becomes transparent, and is usually regarded as a membrana nictitans, as it is provided with a peculiar motor apparatus. In the Anura, the eye possesses not only the ordinary four recti muscles and the two obliqui, but there is a retractor bulbi. The Frogs and probably other Anura, possess a Harderian gland; but no lachrymal gland has been observed. The sclerotic may be chondrified, but it is not ossified, There is no pecten. With regard to the organ of hearing, the membranous labyrinth is enclosed between the pro-otic bone, in front, and the representatives of the opisthotic and epiotic (usually confounded with the exoccipital), behind. The fenestra oval is always occupies a space in the line of junction of the pro- otic with the posterior ossification, whether it be occupied by a broad unossified space, as in Menopoma, or the two bones be ankylosed together, as in Siren, Triton, and old Frogs. The stapes is more or less ossified, and its outer face is frequently provided with a styliform appendage, in the Urodela. In the Urodela (which have no tympanic cavity), a ligament passes from the stapes to the suspensorium, and there is no columella auris. The like absence of columella auris and of a tympanum obtains in several Anura. Duges states that the columella is wanting in Bombinator and Pelobates 1 (Reclierches, p. 41), and the absence of the colu mella auris, as of the tympanum and Eustachian tubes, has since been noticed in Telmatobius, Phryniscus, Atelopus varius, and Brachycephalus ephippium (Stannius, op. cit., p. 61). In the higher Anura, there is a complete tym panum, with Eustachian tubes, and a cohtmella auris, which extends from the stapes to the membrana tympani. The tympanic membrane is either quite similar to the rest 1 Pelobatea, however, has an extremely minute, ossified, columella a/urio. of the integument or markedly different from it. In some genera (e.g. Rana), the tympanic membrane is set in a frame of cartilage. In Pipa and Dactyleihra, the Eusta chian tubes, as already remarked, have a common opening, and the columella is very peculiar. Recent investigations make it probable that the Am2)hibia possess a rudimentary cochlea. 2 Whether the opening, which in the Frogs has been described as a fenestra rotunda^ is really of that nature, ia doubtful. The Reproductive Organs. The ovaria and testes are attached to the dorsal wall of the abdominal cavity, in the immediate vicinity of the kidneys, by the mesoarial and mesorchial folds of the peritoneum, which invest them. The ovaria, when fully developed, become hollow, and in the Anura their internal cavities may be divided by septa. The oviducts are long, usually more or less convoluted, tubes, which open posteriorly into the cloaca ; while, ante riorly, their funnel-shaped apertures lie in the anterior part of the abdomen, sometimes, as in the Frogs, as far forward as the root of the lung. Their walls are glandular, and secrete a viscid substance which invests the ova in their passage down the oviduct. In the male Urodela, the persistent Wolffian duct, al ready mentioned, occupies the position of the oviduct in the female, and the vasa e/erentia, after traversing the kidney, open into it. This duct persists in Bombinator igneus and Discoglossus pictus ; but in the male Anura, in general, the greater part of it is obliterated, only so much remaining as plays the part of ureter and vas deferens. In the Urodela accessory glands open into the cloaca, and in Triton there is a rudimentary copulatory papilla. Some female Urodela are provided with receptacula seminis. In the terrestrial Salamanders and in the anurous Rhinoderma, Gayi the young are developed within the dilated uterine terminations of the oviducts. In Pipa the eggs are deposited on the back of the female, and the integument grows up round each, and encloses it in a cell, in which it undergoes its development. In some tree-frogs (Nototrema and Opis- thodelpliys) the eggs are received into a sort of marsupial pouch formed by an up-growth of the margins of the dorsal integument, which, when complete, has a small posterior aper ture. On the other hand, it is the male Alytes obstetricans which twists the strings of eggs laid by the female round his hind-legs, and, thus cross-gartered, retires into seclusion until the young are ready to be hatched, when he resorts to the water in which the tadpoles are to perform their further metamorphoses. Development of the Amphibia. The yelk of the ovum undergoes complete division, in which respect the Amphibia agree with the Pharyngobranchii, Marsipobrcmchii, and Mammalia, and differ from other Vcrtebrata.; though it must be remembered that tho process of yelk division in the Ganoidei and Dipnoi is not yet known. Except in some viviparous species, the embryo, when hatched, 13 pisciform and apodal; and three pairs of external gills, which become more or less branched, are developed from the first, second, and third branchial arches. In the larval Triton a very singular elongated appendage makes its appearance on each side of the head, in front of the branchiae ; 3 and in the tadpole two eminences of the ventral integument, with glandular terminal faces, are developed one oil each side of the middle line, behind the mouth. The larvro of Dactyleihra have two long tentacles attached near the angle of the mouth. An opercular fold of the integument grows back from each hyoidean arch, and the two are connected by a free fold of the sub- gular integument. The limbs make their appearance as buds from the sides of the body, the anterior pair appearing first. The anterior limbs attain a considerable size before the posterior pair are developed in Triton; but, in the Frogs, the posterior limbs grow much faster than the anterior, which long remain inconspicuous and hidden. In the Urodelc larvae, teeth are very early developed in the pre- maxillary, maxillary, vomerine, palatine, splenial, and dentary 2 See Hasse, Die vergleichende Anatomic dcs hauiigen Gehurorgauet, Leipsic, 1873. 3 The larval Sircdon has no such hyoidean (?) appendage,