Amphibia - General Morphology

AMPHIBIA - GENERAL MORPHOLOGY Integument.—The skin of modern Amphibia may be rough and dry or smooth and moist, but it is never covered with scales as in nearly all reptiles. Many coecilians, however, have rings of minute scales hidden in the transverse folds of their skin. Aquatic or fossorial Amphibia, which rely to a large extent on their skin for respiration, usually have a thinner and smoother integument than terrestrial forms, but many exceptions to this rule occur, particularly in the Pipidae and Bufonidae. The fossil labyrintho donts, phyllospondyls and lepospondyls were frequently well armoured with scales, especially on the ventral surface where these scutes of varying shapes and sizes were arranged, for the most part, in orderly rows con verging toward the mid-line. In a few salamandrids and many trop ical Salientia a secondary de posit of bone forms in the deeper dermal tissues and may pro duce encrustations ankylosed to the underlying bones. The bizarre casques of Pternohyla, Triprion, Cerathyla, etc. are of this character (fig. 7). Less con spicuous bony covers to the skull appear in the spade-foot toads and various hylids, particularly the marsupial frogs. Similar bony growths extend to the derm of the back in various Salientia and in rare instances (Brachyce p/ialus) may be ankylosed to the vertebrae. The epidermis cover ing warts or spines may be more or less cornified in modern Amphi bia and in various urodeles (Desrnognathus, Necturus, etc.) that on the digit tips is similarly modified. In a few hynobiids thick horny pads occur on the palms and soles, while in the larvae of others, the epidermis of the digits is extended into pointed claws. Similar horny claws occur on the three inner toes of the African pipids, and may have been found on all the digits of some extinct Amphibia, for in one of the most primitive labyrinthodonts the terminal phalanges have the form of curved claws.

The integument of modern Amphibia is further distinguished from that of reptiles in being highly glandular. There are two main types of glands : the granular and the mucus. The first may be broadly distributed over the dorsal surfaces or massed in f olds or warts on the sides of the body. Its secretion is creamy, very irritating to mucus membranes and poisonous when taken internally. The mucus glands are more uniformly distributed over the body. They produce a slime less irritating to the nasal passages and conjunctiva but, nevertheless, toxic when injected into lower animals. Amphibia differ enormously in both the quantity and kind of integumentary secretion. Some species, such as the toads, secrete only when mistreated, while others, such as Bombina and particularly the American Plethodon glutinosus, produce a copious irritating or sticky secretion when handled. Closely related species may differ considerably in their secre tions. Brazil and Vellard report that the South American Cera top/irys americana has a virulent poison while the much larger and more brightly coloured C. dorsata possesses an innocuous skin secretion. Brightly coloured Amphibia are frequently not more poisonous than their dull coloured relatives, and some, such as the red-legged Leptodactylus pentadactylus may lack a poison ous secretion. Frequently the secretions are odorous, certain closely related forms of Rana, Bu f o and Hyla being readily dis tinguished by their odour alone.

The skin of the larva differs from that of the adult in that the glands are unicellular and the epidermis consists of only two layers of cells. Larvae approaching metamorphosis and most perennibranchs have a thicker epidermis equipped with multi cellular glands. The superficial layer of epidermis is periodically shed by the adults and usually in one place. The first shedding of the entire epidermis occurs normally at metamorphosis, but per ennibranchs, such as Cryptobranchus shed their skin in one piece before they lose their external gills, and the larvae of some sala manders shed their skin in minute pieces during part of their aquatic life.

Pigmentation.

The coloration of Amphibia is due in part to a diffuse pigment distributed throughout the integument, but chiefly to a number of cellular elements of which three main types may be recognized : first, white cells which owe their colour to a dense deposit of guanine (a substance allied to the urates) ; secondly, the yellow cells which contain droplets of oil; and third ly, the dark brown or melanin filled cells. White cells are often clumped together in small areas under the epidermis producing the white flecks on the back of many salamanders, or the white spots in some tree frogs. Other guanine-filled cells lie closely associated with the yellow and brown cells directly under broad areas of the epidermis and give the characteristic ground colour of these regions. Green is a mixed colour due to the blue rays of light reflected from the small guanine or "turbid cells" mixing with the yellow tone in the overlying yellow cells to produce a green. The blue reflections of the turbid cells are swamped by the white reflections of the subjacent tissues when the melano phores or dark cells are contracted. Brown is produced by the complete enclosure of the turbid cells by the expanded melano phores. The latter expand to surround the turbid cells when stimulated by cold, moisture and rough surfaces, or by hormones produced by the pars intermedia of the pituitary gland. The colour tone of a frog or toad at any one time may be due to either stimulations received through the skin or eyes or to the physio logical (emotional) state of the individual.

Organs of Nutrition, Respiration, and Reproduction.— As these organs are only rarely used in defining natural groups of Amphibia they may be discussed here briefly. Amphibia are primarily carnivorous, only the larvae of the Salientia being more or less herbivorous in their diet. The intestine is therefore short, except in the tadpoles. The tongue is more developed in the primitive forms and may be rudimentary or absent in the aquatic species. The development shows that the urodele tongue arises out of two parts of which the hinder is homologous to the tongue of fishes, while the anterior and lateral part is a glandular fold which has been added to the first part during evolution.

Although the Amphibia have not evolved from the Dipnoans, they show many similarities in their venous and arterial systems. The heart of salamanders shows various modifications according to the extent that the lungs are used in respiration. Many urodeles living near mountain streams have lost their lungs and respire entirely by their skins and well-vasculated throats. The entire family Plethodontidae is lungless. Many other salamanders use their lungs primarily as hydrostatic and not respiratory organs. Salamanders are mute or endowed with a very poor voice. Most Salientia have a larynx modified and equipped with vocal organs. Frogs call chiefly to attract mates in the spring.

Fertilization in the Amphibia may be external or internal. Most salamanders, except the Hynobiidae and Cryptobranchidae, are equipped with a spermatotheca, a series of tubules in the roof of the cloaca where the sperm remain after copulation until the time of fertilization. In these salamanders the sperm mass is emitted by the males in the form of spermatophores, small tufts of sperm attached to a gelatinous base. Courtship in the salamanders is primarily directed towards exciting the female sufficiently to ensure her picking up the spermatophore with the cloacal lips. The sperm then make their way into the spermatotheca by their own efforts. The Gymnophiona are equipped with a protrusible cloaca which is used as an intramittent organ. The male Ameri can liopelmid frog, Ascaphus, has a movable extension of the cloaca. In these forms fertilization is internal, as well as in cer tain little-known African frogs which have no special apparatus for transmitting sperm. The gonads are variously modified in different groups permitting a greater freedom of the seminal ducts from the kidneys. In Bufo a rudimentary ovary, Bidder's organ, is found in front of the testis and when the latter is re moved it develops into a functional ovary. Sex reversal has also been recorded in both frogs and salamanders. (See SEX.) Life History.—Most Caudata and Salientia lay their eggs in the water, where they hatch into larvae or tadpoles destined to live a more or less extended period in this element before meta morphosing into terrestrial adults similar to their parents. The larval state seems to be an old inheritance of the Amphibia hand ed down from their crossopterygian fish ancestors. Even the most primitive Amphibia underwent a metamorphosis, for the fossilized remains of both larval labyrinthodonts and branchio saurs are known.

Metamorphosis represents a period of tremendous change in both the morphology and physiology of Amphibia. It is marked by such external changes in urodeles as the reduction of the ex ternal gills, the loss of the tail fin, the shedding of the larval skin, the formation of eyelids, etc. But fundamental changes in skull form and throat musculature take place. In Salientia meta morphosis is indicated by a freeing of the forelimbs by autolysis of local areas in the operculum, by radical changes in the shape and structure of the head, and by absorption of the tail. One of the re cent achievements of physiology has been the demonstration that the changes of metamorphosis are produced by the thyroid hor mone, or at least this hormone working in conjunction with that of the anterior lobe of the hypophysis. A review of the physiology of development cannot be given here, but reference may be made to the summaries of Remy ("Les secretions internes et les meta morphoses," Amer. Sci. nat. Zool. [Io], vii.), Uhlenhuth ("The internal secretions in growth and development of amphibians," Amer. Nat., 1921) and Allen ("Influence of the hypophysis upon the thyroid gland in amphibian larvae," Univ. Calif. Publ. Zool., xxi. 1927). (See ENDOCRINOLOGY.) Certain groups of urodeles, such as the Cryptobranchidae, Sirenidae and Proteidae, fail to metamorphose. This phenome non appears as a variation in the life history of many urodeles, particularly in those species passing their larval period in cold water. Neoteny in the latter case is due to the failure of the thyroid to function, but the causes of arrested development in the case of groups unknown as metamorphosed individuals is more complex. In considering the relations of the perenni branchs, it is important to compare their organization with that of the larvae of other groups, not with that of the adults. Neoteny is not a new development in the Amphibia. It is found even in the Permian Dwinosaurus.

Some salamanders and many tropical Salientia produce large yolked eggs which they usually lay on land, although a few re tain them within the oviducts until the young are born more or less fully developed. Only two frogs are viviparous and both are rare East African species (Nectophrynoides tornieri and N. vivi para). The breeding habits of the Amphibia are often elaborate and it is noteworthy that one may trace within the group a grad ual evolution in many of the instincts associated with courtship and egg laying.

The larvae of the Amphibia frequently exhibit specializations which are of phylogenetic or at least taxonomic importance. The larvae of the marsupial frog and its allies possess enormous bell shaped gills. All brevicipitid tadpoles, except those of certain South African genera which probably had an independent origin, have the same toothless, protrusible buccal apparatus. Where the life history is very specialized both larval modifications and breed ing habits will tend to indicate the relationships of the forms. Thus, only Protopipa and Pipa carry their eggs in individual der mal chambers on the maternal parents' back; Phyllobates and Dendrobates are the only genera which transport to the streams their unspecialized tadpoles on the males' back ; Cryptobatrachus, Cerathyla, Gastrotheca and Amphignathodon, the only ones which carry their eggs in a single mass on their backs (either ex posed or enclosed in a dermal f old) . The genera in each of these groups differed one from the other in matters of dentition and, hence, according to earlier classifications, would not be considered closely related. But recently when both anatomical and life history data were considered in further detail it was recognized that each of these groups is in fact a natural one.

Geographical Distribution.

Distribution of the urodeles does not agree with that of the Salientia and the latter is unlike that of the coecilians, for each of these groups probably spread from different centres, were affected by different barriers, and subjected to diverse conditions of travel. The present dis tribution of the Amphibia gives evidence of former union of the northern continents. Cryptobranchid salamanders are found only in eastern Asia and eastern North America. Proteid salaman ders are found only in Europe and North America. One genus of plethodontids, Hydromantes, is found only in the two latter regions. One genus of salamandrids, Triturus, is holarctic, al though a closely related form has apparently managed to work its way south of the Sahara. Rana and Bufo have closely related species in North America, Europe and Asia. The present distribu tion of the Amphibia also necessitates an early Tertiary connection between North and South America, for the plethodontid genus, Ensatina, is now found in Argentina, also the bufonid toads have undergone an enormous adaptive radiation in the latter continent. The ranids and some brevicipitids of South America may have reached the continent after the present Panamanian connection was formed, but the occurrence of several endemic genera of the latter group would favour an early migration for this family as well. The present distribution of the Pelobatidae, Bufonidae, Ranidae, Polypedatidae and Brevicipitidae apparently demands a land bridge between Asia and Africa during part of the Tertiary and possibly a direct connection with the Seychelles. Pelobatids are not found in Africa, but in Asia, some of the East Indies, Europe, and the Seychelles. The bufonid Nectophryne is found in the oriental region and the African rain forest. The African Chiromantis is closely allied to the oriental Polypedates. Rana must have been in Africa for some time, for it has split up into a number of endemic subgenera. Madagascar is frequently claimed to have derived its fauna directly from the oriental region. Pocy pedates is found in both regions. The Madagascan Dyscophus is closely allied to the Asiatic Calluella. But Madagascar lacks coecilians and Bufonidae. Its ranids and brevicipitids have under gone an extensive local specialization which would suggest isola tion. Further, it possesses Hyperolius, Megalixalus and Rana in common with Africa. If these recent and aggressive genera have gained access to Madagascar without the aid of a land bridge it is highly probable that brevicipitid and ranid stocks may have done the same at an earlier period when other genera formed the dominant African fauna.

It is frequently claimed that the present distribution of the Amphibia affords some of the best evidence of the existence of former land bridges in the southern hemisphere. Coecilians are tropicopolitan; pipid toads occur only in tropical Africa and South America. Hence, many zoogeographers would postulate a land bridge across the mid-Atlantic. There is good evidence that in the case of other primitive groups extensive migrations have occurred in the northern region without leaving any fossil re mains. For example, one genus of liopelmids is found to-day in North-western United States, the other in New Zealand, and con sequently liopelmids must have occurred in the intervening area at one time. Salientia do not make good or frequent fossils, but the occurrence of toothed bufonids ("leptodactylids") in Aus tralia, Africa and South America has led Hewitt to assume a radiation of toads from the south to Asia and the two latter con tinents along three theoretical land masses existing in Mesozoic times. Such speculations have little in their support and it is far simpler to assume that all families of Amphibia, except the neotropical Brachycephalidae, arose in one of the northern con tinents and migrated south along existing continental land masses. To-day ambystomids, sirenids and amphiumids are endemic to North America and may have originated there. The large family of Plethodontidae are found chiefly in that continent. Hynobiids are known only from Asia. Salamandrids have their headquarters in Europe, although Asia, North Africa and North America have received a few species. Discoglossids are palarctic, pelobatids mainly holarctic. Bufonids, hylids, ranids and brevicipitids have a world-wide range, but exhibit certain remarkable lacunae in their ranges. Thus, hylids are not found in the Ethiopian region (with a single possible exception) nor in most of South-eastern Asia and the adjacent islands; ranids, except the modern genus Rana, are absent from South America and Australia. The latter also lacks brevicipitids, except in the north-east. Papua has an amphibian fauna essentially like North-eastern Australia. It possesses bufonids, hylids, ranids and brevicipitids, but no pelobatids. The bufonids have undergone an adaptive radiation in Australia, while the brevicipitids did the same in New Guinea. Economic Value.—Toads are of great use to farmers as they are indiscriminate feeders on insect life. In cases of an insect plague toads feed on the dominant insect and tend to restore the balance of nature. Frogs and toads are used for food in many parts of the world, but successful frog farms are rare. Frogs re quire from two to five years to reach maturity, and large quanti ties of living insect food are needed during this period. Hence, in America almost the entire frog crop is obtained in the wild state.

Dried frogs and salamanders are used for medicinal purposes in various parts of the world, particularly in the Orient. Their value as a cure is chiefly psychological. Probably the greatest use of Amphibia to humanity is their martyrdom to science. Frogs and salamanders both in adult and young stages have long been recognized as ideal laboratory animals. Some of our most fundamental discoveries in developmental mechanics (See Ex PERIMENTAL EMBRYOLOGY), endocrinology (q.v.) and general phy siology (q.v.) have been made with amphibian material.

BIBLIOGRAPHY.—General Works.—G. A. Boulenger, Catalogue of Bibliography.—General Works.—G. A. Boulenger, Catalogue of Batrachia Salientia S. Ecaudata in the collection of the British Museum (1882) , Catalogue of Batrachia Gradientia S. Caudata and Batrachia Apoda in the collection of the British Museum (1882) , The Tailless Batrachians of Europe, i.–ii. (1897-98), Les Batraciens et principale ment ceux d'Europe (191o) ; E. D. Cope, "The Batrachia of North America," Bull. U. S. Nat. Mus., No. 34 (1889) ; E. Gaupp, Ecker and Wiedersheims Anatomie des Frosches (Braunschweig, 1896-1904) ; H. Gadow, "Amphibia and Reptiles," Cambridge Natural History viii. (toot) ; O. Hertwig, Handbuch der vergleichenden and experimentellen Entwicklungslehre der Wirbeltiere (Jena, 1906) ; M. C. Dickerson, The Frog Book: North American Frogs and Toads, with a Study of the Habits and Life Histories (1906) ; F. Werner, Brehm's Tierleben, 4th ed., iv. "Lurche and Kriechtiere" (1912) ; J. Versluys, Amp'hibia, Handworterbuch der Naturwiss. i. (1912) ; S. J. Holmes, The Biology of the Frog (1912) ; R. Lydekker, J. T. Cunningham, G. A. Boulenger and J. A. Thomson, Reptiles, Amphibia, Fishes, and Lower Chordata (1912) ; D. M. S. Watson, "The evolution and origin of the Am phibia," Phil. Trans. Roy. Soc. (London) (B) ccxiv. (1914) ; O. Abel, Die Stamme der Wirbeltiere (1919) ; J. G. Kerr, Text Book of Embryo logy, ii., "Vertebrata" (1919) ; E. R. Dunn, "The Salamanders of the family Hynobiidae," Proc. Amer. Acad. Arts and Sci., lviii. (1923), The Salamanders of the family Plethodontidae (Northampton, Mass., 1926) ; K. A. Zittel, Grundziige der Palaeontologie, neu bearbeitet von F.

Broili and M. Schlosser (1923) ; G. K. Noble, "The evolution and dis persal of Frogs," Amer. Nat. (1925), "The value of life history and data in the study of the evolution of the Amphibia," Ann. N. Y. Acad. Sci., xxx. (1927) ; E. Perrier, Traite de Zoologie, vii. "Les Batraciens" (1925) ; J. E. W. Ihle, Vergl. Anat. der Wirbeltiere (1927). (G‘K. N.) AMPHIBIAN PLANE: see AERONAUTICS and SEAPLANE.