The ova are shed into the coelome, whence they are taken up by the abdominal apertures of the Miillerian ducts, which are close to the anterior ends of the kidneys in Gymnophiona, at the roots of the lungs in other Amphibia. They are straight in Gymnophiona, somewhat convoluted in Urodela, and especially so in Anura at the breeding season. In viviparous Salamanders their distal portion is dilated and functions as uterus. It is also dilated and acts as a reservoir for the albumen-coated ova in Anura. The two ducts open separately on the dorsal wall of the cloaca, except in Alytes and Bufo, where they unite and open by a common aperture. In both male and female Toads (Bufo) an undeveloped and cellular remnant of the genital rudiment lies at the anterior end of the sexual glands, and is known as Bidder's organ1. 'Fat bodies' of unknown function are found in all Amphibia, laterally placed to the sexual glands in Gymnophiona: between them in Urodela and at their anterior ends in Anura. The cloaca is partially eversible in the male Gymnophiona, and is probably used as an intromittent organ.
Specialised copulatory organs are not present.
1 For an account of this remarkable organ, see Knappe, M. J. xi. 1886. It tends to disappear in the female sooner or later. Its cells closely resemble ova and are inclosed by follicle cells, but isolated cells here and there have been observed to form spermatozoa. It occurs in hermaphrodite individuals.
Caecilia compressicauda among Gymnophiona is viviparous, as are Salamandra maculosa and S. atra. Other Amphibia are oviparous, and the ova are generally laid in water, either in floating masses (Anura) or affixed to water-plants (Triton, Axolotlamong Urodela). Certain exceptional cases may be noted among Anura. The ova are stored in a dorsal sac opening above the cloaca in Notodelphys and Nototrema: in cavities one for each ovum, formed by hypertrophy of the dorsal skin in Pipa, the male spreading them on the backs of the female. They are affixed to leaves of plants in one or two instances, e. g. by Hylodes martinicensis: are carried by the male Alytes obstetricans twisted round the hind-limbs until the Tadpoles are ready to escape, and by the male Rhinoderma Darwinii in the enlarged croaking sacs l. Impregnation is external in Anura. The male Urodele, so far as is known, deposits a spermatophore, formed by the cloacal and pelvic glands (?), which is taken up by the female and therefore impregnation is internal, as it must be in Gymnophiona. There is a sexual congress in Anura, and in one Urodele at least (Glossoliga Hagenmülleri). The ova are enveloped in albumen secreted by the oviduct. They are usually small.
Segmentation is as a rule complete, but in some instances there is a large amount of yolk and a yolk sac, as in Teleostean fish, e. g. Epicrium glutinosum, Pipa, Alytes. Segmentation is unequal. The epiblast consists of two strata, an epidermic and a nervous, from the first in Anura, a condition attained at a later period in the Newt. There is a metamorphosis, the degree of which varies. The larva has usually a fish-like aspect: the head not well marked off from the body: a long tail with azygos fin: three pairs of external gills: no limbs except mere indication of the fore-limbs in the Newt: adhesive organs in the Anura, and a pair of supporting processes below the neck in the Newt. The young Caecilia compressicauda, Salamandra atra, Pipa and Hylodes martinicensis have the adult form when they come into the world. The two first named have an intra-uterine development, the latter of the two having long gills, which are lost if the young are transferred to water, a new and shorter set being developed. Pipa passes through its metamorphosis in the cavity of the skin in which the ovum lies. Hylodes has an intra-ovular metamorphosis, and in two species of Nototrema the young quit the dorsal pouch in a perfect state.
Epicrium glutinosum loses its external gills before it is hatched, and the larva wanders from the spot in -the earth where the eggs are laid and leads an aquatic life for a time. It swims by means of its tail, which is provided with a fin. The tail is lost in the adult as in other Gymnophiona. The Anuran Tadpole also loses its tail by absorption. The changes which take nlarp in the metnmnrnhnsis are nrofound in the Anura and higher Urodela.
1 See remarks and a table by Boulenger to a paper by von Ihering in A. N. H. (5) xvii. 1886.
And have been indicated in part above. For details larger works must be consulted.
The Alpine Triton alpestris, one of the Salamandrina, has been observed to attain sexual maturity, indicated by external signs and by maturation of ova and spermatozoa, at a time when the branchiae are still in functional activity, and the characters of the skeleton show the animal to be really in a larval state. The Axolotl (Siredon pisciformis) has long been known to be competent to sexual functions whilst in a perennibranch condition, a condition in which many individuals remain. Other individuals undergo a change into the Salamandrine Amblystoma mexicanum. This change takes place in its natural habitat, and has been also induced artificially.
Some Amphibia, but not Anura, show great power of repairing injuries and of reproducing destroyed or amputated organs. Many of them possess protective coloration which is intensified by the state of distribution of pigment controlled by the nervous system. The Anuran larva feeds on vegetable matters. The adult is carnivorous in all instances, insects, worms, snails, etc, and even higher animals forming the prey. The Amphibia are absent from nearly all oceanic islands. The Anura are widely distributed: the Urodela are limited to the temperate parts of the northern hemisphere, and are especially numerous in North America. No fossil Gymnophiona are known. Anura and Urodela occur in Miocene strata. The extinct Stegocephali are found in the older Mesozoic strata (Trias) and in the more recent Palaeozoic (Permian, Carboniferous).
The class Amphibia may be divided into orders as follows:
Snake-like in form and subterranean in habit; no limbs nor tail; dermic scales imbedded in the integument. Caecilia is found in West Africa, Malabar, and South America; Siphonops in Brazil and Mexico; Epicrium in Ceylon and the Khasya Mountains; and Rhinatrema, which has no tentaculiferous fossa in Cayenne. There are certain resemblances between this group and the family Aistopoda among Stegocephali; see Fritsch (infra).
Body elongated; as a rule four short extremities and persistent tail with azygos fin.
Eyes small, with or without a circular eyelid-like fold; with biconcave vertebrae and large remnant of notochord. Aquatic in habit.
(a) Perennibranchiata. Three pairs of persistent gills. Siren, with fore-limbs only, comes from South-east States of North America; Proteus from caves in Carniola and Dalmatia; Menobranchus from East States of North America (said to change into Batrachoseps); the Axolotl, Siredon, from Central America; it changes sometimes into Amblystoma Mexicanum. (β) Derotremata. Gills caducous; as a rule a persistent gill cleft. Amphiuma (=Muraenopsis) comes from south part of North America; Menopoma (=Protonopsis) from Ohio and Alleghany Rivers; and Sieboldia (= Cryptobranchus) from Japan.
Branchiae caducous; gill clefts closed in adult; eyelids; opisthocoelous vertebrae in higher forms. Spelerpes, Amblystoma, Triton, Lisso-triton, Salamandra, etc.
Body compressed; tail and branchiae lost; hind-limbs of great length, and used for leaping. Rana, Bufo, etc.
A tail; paired supra-occipital bones, supra-temporals, and post-orbitals present; generally also both pre- and post-frontals and orbital ring of bones; pubes well ossified; extinct. Extend from the Permian to the Trias.
Gymnophiona, Wiedersheim, 'Anatomie der Gymnophionen,' Jena, 1879. Leydig, Z. W. Z. xviii. 1867-69. Development of Caecilia, Peters, Monatsberichte Akad. Berlin, 1874-1875; of Epicrium, Sarasin, P. B. and E. F., Arb. Zool. Zoot. Inst. Wurzburg, vii. 1885.
Urodela. Anatomy of Salamandra perspicillata and Geotriton fuscus, Wiedersheim, Annali del Museo Civico di Storia Naturale di Genova, vii. 1875. Ambly-stoma punctatum, Id. Z. W. Z. xxxii. 1879. Pleurodeles Waltlii, Fraisse, Arb. Zool. Zoot. Inst. Wurzburg, v. 1882; skeleton, Wiedersheim, J. Z. xiv. 1880. 'Molche der Wurtemburgischen Fauna,' Leydig, A. N. 33, 1867 (sep. Berlin, 1868). Lateral line, Malbranc, Z. W. Z. xxvi. 1875; Merkel, Endigungen der sensibeln Nerven in der Haut der Wirbelthiere, Rostock, 1880. Skull, W. K. Parker, Ph. Tr. 167, 1877; Tr. L. S. (2), 1882; Tr. Z. S. xi.; Wiedersheim, M. J. iii. 1877; Stohr, Z. W. Z. xxxiii. 1880. Cartilage end of back-bone, Fraisse, Z. A. iii. 1880. Cloacal and pelvic glands, Blanchard, Z. A. iv. 1881. Impregnation of Axolotl, etc. Gasco, Z. A. iv. 1881. Change of same into Amblystoma, (I) natural, Velasco, Biol. Centralbl. ii. 1882-83; (2) artificial, M. von Chauvin, Z. W. Z. xli. 1885, and xxvii. 1876; cf. von Siebold, ibid. vol. cited last; cf.
Weismann, Studies in Theory of Descent (transl. by Meldola), pt. 3, 1882. Sexual Urodele larvae, Von Siebold, Z. W. Z. xxviii. 1877. Larva of Salamandra atra, M. von Chauvin, Z. W. Z. xxix. 1877. Regeneration of lost parts, Gotte, Uber Entwickelung und Regeneration des Gliedmassenskelets der Molche, Leipzig, 1879; Fraisse, Tagesblatt der 52 Ver-samml. Deutsch. Naturf. Baden-Baden, 1879, p. 223; Id. Biol. Centralb. iii. 1883-84; cf. Wiedersheim, M. J. ii. 1876.
Anura. See p. 78-9, 82-3. Organs of adhesion of larva, Stohr, SB. Phys. Med. Gesellsch. Wurzburg, 1881, p. 118; Niemiec, 'Les ventouses,' etc.; Recuel Zool. Suisse, ii. (I), 1885.
Stegocephali, Fritsch, Fauna der Gaskohle und der Kalksteine der Permforma-tion Bohmens, Prague, i. 1883; ii. pt. 1, 1885; pt. 2, 1886; cf. Labyrinthodontia, Brit. Association Reports, 1873, p. 225; 1874, p. 149; Cope on Permian Batrachia, American Naturalist, xviii. 1884.