Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/795

Rh AMPHIBIA 751 the long extinct Amphibia of the Carboniferous epoch. In other respects, the lower Amphibia approach the Chimcerce, the Ganoidei, and the Dipnoi very closely ; while, in their development, they present curious approximations to the Marsipobranchii. With respect to the primary subdivisions, or orders, of the class Amphibia, no one can doubt the propriety of the separation of the recent forms into what may be broadly termed Newts (Urodela} ; Frogs and Toads (Anura] ; and Coecilice (Peromela) effected by Dume ril ; while all that is known of the organisation of the extinct Amphibia of the newer Palaeozoic, and older Mesozoic, formations tends to show that they form a fourth natural assemblage of equal value to each of the others. The names of Urodela and Anura, given to the first two of these divisions, are undoubtedly open to criticism ; but if well-understood terms, which have acquired a definite scientific connotation, are to be changed whenever ad vancing knowle.l^e renders them etymologically inappro priate, the nomenclature of taxonomy will before long become hopelessly burdened ; and, to set a good example, the names of Urodela, Anura, Peromela, and Labyrintho- donta are adopted here for the four orders of the Amphibia, even although it be true that the Labyrinthodonta do not all possess the dental structure on which the name was founded ; though there is reason to believe that some Labyrinthodonts were devoid of limbs, or peromelous ; that the Anura are not more tail-less than are the Peromela ; and that the tails of the Urodela are not more conspicuous than were those of the Labyrinthodonts. odela. The URODELA are Amphibia with elongated bodies and relatively short limbs, devoid of scales or pectoral plates, with numerous prae-caudal vertebrae, and with amphiccelous, or opisthoccelous, vertebral centra. The hyoidean arch re mains connected with the suspensorium throughout life, and its cornua are large in proportion to its body. The mandible is dentigerous. There are one or two pairs of limbs, the pectoral arch and limbs being always present. The manus never possesses more than four digits. The bones of the antebrachium and of the cms remain distinct, and the tarsus is not elongated. So far as the spermatozoa are known, they are elongated filaments with a vibratile fringe. The larva develops external gills only; and, except Siren, none are known to possess, at any time, a horny masticatory apparatus. 1 ura. The ANURA have relatively short and broad bodies, and both pairs of limbs are constantly present, the hinder being the longer and stronger. There are no scales, nor pectoral plates, but ossification sometimes occurs in the dorsal integument. The vertebrae vary in character, but are usually precocious. The prae-sacral vertebrae never exceed nine in number, and the caudal portion of the verte bral column is represented by a peculiar styliform coccyx. The hyoidean arch detaches itself from the suspensorium, and almost always becomes connected with the pro-otic region of the skull. The cornua are usually slender, as compared with the broad body of the hyoid. The mandible is almost always devoid of teeth. The bones of the ante brachium and of the crus early ankylose, and the astragalus and calcaneum are much elongated. The manus has a rudimentary fifth digit. Except in Bomlinator, the sperma tozoa have, flagelliform appendages, like those of ordinary Vertebrata. The larvae develop first external, and after wards internal, gills, and, so far as is known at present, are provided with deciduous horny masticatory plates. The gill 1 This circumstance appears to have been remarked only by Miiller. Speaking of the larvae of the Salamanders, he says&quot; Sie haben nicht den Hornschnabel der Froscblarven.&quot; (Beitrag zur Anat. der Amphi- bien, p. 209.) Dmn4ril and Bibron affirm the contrary (op. cit., t. ix. p. 1C). apertures are closed by the growing over them of an oper- cular membrane. The PEROMELA have snake-like bodies, totally devoid of Peromel limbs and limb arches. In most, the integument is pro vided with transverse rows of imbedded cycloid scales, but there are no pectoral plates. The vertebrae of the trunk are very numerous, and are amphiccelous ; those of the caudal region are very few, and are free. The hyoidean arch is attached neither to the suspensorium, nor to the skull; its cornua are very slender, and no distinct body is developed ; it is followed by several slender, hoop- like, branchial arches. The mandible is dentigerous. Nothing is known of the early stages of development; but Miiller discovered branchial clefts, with rudimentary bran chial filaments, in young Ccecilice. The LABYRINTHODONTA for the most part resembled the Labyrin Urodela in the proportions of the tail and limbs to the donta. body, but some (as Ophiderpeton) were serpentifonn, and apparently apodal ; no raniform Labyrinthodonts have yet been discovered. The vertebras are amphiccelous. The mandible is dentigerous. The bones of the antebrachium and crus remain distinct, and the tarsus is not elongated. The manus and pes appear to have been pentadactyle. Three sculptured pectoral plates and a peculiar dermal armour of small scales, confined to the ventral face of the body, are present in many genera. Nothing is known of the early stages of development, but the ycnrngArchegosauria appear to have possessed ossified branchial arches. In giving a sketch of the organisation of the Amphibia, it will be necessary to enter much more fully into the characters of the SKELETON than into those of the other systems of organs. The Vertebral Column, Ribs, and Sternum. Leaving the extinct Archegosauria aside for the present, all the Amphibia possess well-ossified vertebrae, the arches of which, in the adult condition (except, perhaps, in some Labyrinthodonts), are not separated by a neuro-central suture from the centra. The latter may be amphicoelous, as in the lower Urodela, the Peromela, and the Labyrinthodonta; or opis- thocoelous, as in the higher Urodela and some Anura (e.g., Pipa and Bombinator) ; or proccelous, as in the majority of the Anura (with the exception of the eighth vertebra, which is usually amphiccelous ; and of the ninth, which commonly has one convexity in front and two behind). In all the recent forms which have been examined, the centra and intervertebral masses contain more or less dis tinct remains of the notochord. The arches of the trunk vertebrae are connected by zygapophyses ; the spinous pro cesses are usually low, but attain a great relative length in the caudal region of some of the Labyrinthodonts (e.g. Urocordylus). Transverse processes are present in all the trunk vertebrae, except the atlas ; they are shortest in the Peromela, longest in the Anura. In most Urodela, these transverse processes, at any rate in the anterior trunk ver tebrae, are divided into two portions, a dorsal and a ventral, which diverge towards their free ends; or, more strictly speaking, these processes are made up of two subequal trans verse processes, a dorsal &quot;tubercular&quot; process, and a ventral &quot; capitular &quot; process. Sometimes this division prevails through out the whole length of the trunk, but, more commonly, the two transverse processes become fused into one, posteriorly. In the long-bodied Urodela (Siren, Proteus, Amphiuma), only a small number of the vertebrae which succeed the atlas present traces of double transverse processes ; further back, the coalesced transverse processes form trihedral pro jections, their dorsal and ventral contours converging instead of diverging, and giving a very characteristic aspect to these vertebrae.