The living representatives of the class Amphibia include salamanders, salamanders, frogs, toads and the caecilians. These animals are sometimes called the Stegocephalians because of the solid roofing of the skull. Based on the morphology of their vertebrae, paleontologists believed that fossil amphibians with stereospondylosis and embolomous vertebrae were not in the amniote lineage.
The urodeles have a greater tendency to show general amphibian class characteristics, compared to the much more specialized Anura. Fossils that can be clearly identified as ostracoderms date from the beginning of the Ordovician. Cephalochordates as we know them today were not the genetic ancestors of the first craniates.
The urochordates are also monophyletic, a sister group to the rest of the chordates (cephalochordates plus vertebrates). Conodonts are extremely common fossils in rocks from the late Cambrian to the end of the Triassic. The most likely ancestors of amphibians were the rhypids that were common in the Permian.
Grooves in the skull of some juveniles carried the lateral line system, which, however, was absent in the adults of the same species.
AMPHIBIA: GENERAL ORGANIZATION
From the dorsal end of each of the three branched arches arises a branched external gill. The skin is soft and usually mucous due to the secretion of the cutaneous glands. The spleen is a hematopoietic organ in anura, found attached to the anterior end of the rectum via a mesentery.
The latter are paired sac-shaped structures located in the front of the body cavity on the sides of the heart. The pitch of the voice is controlled by the level of tension created in the vocal cords. Of the two largest valves, one emerges from the dorsal border and the other from the ventral border.
The conus arteriosus is a tubular chamber oriented obliquely on the ventral side of the right auricle. There has been a reduction in the number of bones and a general flattening of the skull. The atlas is articulated with the skull by a pair of occipital condyles, projections of the exoccipitals.
The massive arch on the dorsal side of the ring is called the neural arch. The olfactory epithelium in terrestrial forms is located in the upper medial part of the nasal passages. Lateral line organs are retained in the perennibranchiate urodeles and in the larvae of the terrestrial forms.
In adult apods, the opistonephros spans most of the length of the coelom and is lobulated. Kidneys are flat, oval, dark red organs, in the posterior part of the coelom. The yellow-orange adrenal gland is located on the ventral side of the kidney.
PARENTAL CARE IN AMPHIBIA
For example, salamander larvae remain in the maternal tract to obtain food after the egg yolk is depleted. Caecilians are oviparous or ovoviviparous, with some forms retaining developing embryos in the fallopian tube and feeding on its lining. A classic example of pedogenesis is the Mexican axolotl, which often reproduces in the larval state (neotenic forms).
Another case of pedogenesis is seen in the Alpine newt, where complete metamorphosis occurs in the habitats of the French and Italian lowlands, whereas the race inhabiting the colder Lombardy lakes is often neotenous. In aquatic enclosures, the Brazilian frog, Hyla faber, protects its offspring by building a basin-shaped nursery in the shallow water bordering a pond (Figure 22 A). The eggs and larvae are thus protected from attack by many insects and fish at least for some time, then heavy rain destroys the wall and the larvae go straight to the water.
In holes near water - An even better way to protect the offspring during the early stages of development has been adopted by the Japanese tree frog, Rhacophorus schlegelli. The male and female bury themselves together in the moist soil at the edge of a ditch near a flooded paddy field, making a hole or chamber a few centimeters above the water level (Figure 22 B). It slopes down towards the water and is later used by the newly hatched larvae that come to the water to complete their development.
The larvae move around in the foam and, having lost their external gills, fall into the water to complete their metamorphosis. In transparent gelatinous sacs in water, Phrynixalus biroi has large eggs enclosed in a sausage-shaped transparent common membrane, secreted by the female and this sac-like structure is left in the mountain streams. Tadpoles carried from one place to another by the male parents. Small South American frogs Phyllobates and Dendrobates and Arthrolepis seychellensis carry well-developed tadpoles on their backs (Figure 22I).
Eggs protected by the male parent covering them with his body. The eggs of Mantophryne robusta are strung together by an elastic, gelatinous envelope. Around the eggs by the male: In Alytes obstetricans, mating and oviposition take place on land, and the eggs are deposited by the female in batches of 2-3, at short intervals. The male lands and the eggs are deposited by the female at short intervals in batches of 2-3.
On the back of the female
The male then ties the eggs into a rope-like structure and wraps them around his legs for protection (Figure 22 D).
Exposed on the belly of the female: The female of Rhacophorus reticulatus carries its eggs on the belly, which bears shallow impressions when the eggs are removed
In the mouth or gular pouch
Short-lived amplexus occurs wholly or partly on land, as seen in Salamandra, Plethodon, and Autodax. In some forms the eggs are small and the larvae hatch quickly and no parental care is observed, but in other examples parental care appears to be as prominent as in anurans. In holes on land or in trees - Autodax lays about 10-20 eggs in a dry hole in the ground or in a hole in a tree, about up to 30 feet above the ground.
The mother or both parents stay in the nest during development to defend the brood and also to provide them with moisture. In a transparent bag in water - Salamandrella keyserlingii deposits its eggs in a jelly-like bag, which is attached at one end to an aquatic plant just below the water surface. The larvae remain in the bag and hatch into an advanced stage of development.
The female coils around the eggs - In Plethodon, the eggs are laid under stones in small clumps, and the mother coils her body around them. The larvae survive on a large spherical mass of yolk and leave the gelatinous egg shell only after losing their gills. Males coil around the eggs - In Megalobatrachus maximus, it is the male parent who coils around the eggs and protects them during the early stages of development.
Female parent carries the eggs on her back or around her legs - In Desmognathus fusca, the eggs are laid in the form of rosary-like cords. The egg strings are then tied around the body many times and the female parent feeds them for some time. Viviparity- Salamandra maculosa mates on land and several months later the female goes to the water and gives birth to 10-50 young which are small in size and resemble the salamander larvae with developed forelimbs.
In Salamandra atra, the young are kept in the womb until metamorphosis is complete. In Ichthyophis glutinosa, the female digs a hole close to the surface in moist soil near water. It then deposits about a dozen large, yellow eggs, 8-10 mm in diameter, and wraps its snake-like body around them (Figure 22 C).