"As far as I know, all extracellular mucuses and gels are secreted in concentrated form and then take up water--a conspicuous example is the jelly mass in which the eggs of a frog are suspended. When swollen, the mass is typically larger than the volume of the gravid female. Slime production by hagfish, according to John Gosline, provides an even more spectacular case." (Vogel 2003:444) Learn more about this functional adaptation.
Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
Barcode of Life Data Systems (BOLD) Stats Specimen Records:28763 Specimens with Sequences:33511 Specimens with Barcodes:21388 Species:2618 Species With Barcodes:2449 Public Records:9310 Public Species:947 Public BINs:1794
When only living taxa are considered, a list of synapomorphies of Anura would include many of the obvious distinctive features of frogs:
Shortened vertebral column (nine or fewer presacral vertebrae)
Presence of a urostyle formed from developing tail vertebrae
Absence of tail in adults
Hindlimb longer than forelimb
Fusion of radius and ulna into a single element
Fusion of tibia and fibula into a single element
Elongate ankle bones (tibiale and fibulare = astragalus and calcaneum)
Absence of a prefrontal bone
Fusion of separate hyobranchial elements into a hyobranchial (=hyoid) plate
A tongue that lacks intrinsic skeletal support from the hyobranchial plate
A tadpole, with keratinous beaks and denticles as larval mouthparts
A single median spiracle in the larva (characteristic of Orton's Type 3 tadpole)
Large subcutaneous lymph spaces between the skin and muscle layer
Two protractor lentis muscles attached to lens
(Milner, 1988, 1993; Saint-Aubain, 1981; Trueb and Cloutier, 1991).
However, the existence of several early frog fossils, such as †Prosalirus and †Notobatrachus, clouds this picture, because many of these characters cannot be assessed in these fossils, either because of incompleteness of fossils or because the features are soft-tissue characters identifiable only in living taxa.
Most people learn about the basics of frog reproduction in elementary school. Frogs lay eggs in water, and the eggs hatch into tadpoles that grow into frogs. Only about half of all frogs follow these exact steps, but there are a few rules of thumb about frog reproduction. All frogs reproduce sexually, and all hatch from eggs.
In almost all frogs, egg fertilization happens outside the female's body instead of inside. The female releases her eggs and the male releases his sperm at the same time. In order to make sure that the sperm reach the eggs, the male and female get into a mating posture called amplexus. The male climbs onto the female's back and clasps his forelegs around her middle. Frogs can stay in amplexus for hours or even days as the female releases as few as one or as many as several hundred eggs.
Sometimes, it's easy to tell male frogs from female frogs. Many species are sexually dimorphic, meaning that there are differences between the bodies and colors of males and females. But in some species, males and females are hard to tell apart. In such species, male frogs often produce a release call when clasped by another male. During mating season, researchers can use release calls to tell which frogs are male and which are female.
All frogs' eggs require moisture to develop, and most frogs abandon their eggs once they're fertilized. But not all eggs incubate underwater or without parental care. A few species carry their eggs in their vocal sacs or their abdomens. Others lay eggs in dry areas and keep the eggs moist with water or urine. Depending on the frog's species and the climate in which it typically lives, the eggs can hatch in a few days to a few weeks.
In a few species, fully formed froglets hatch from the eggs, but most of the time the frog starts its life as a tadpole. While adult frogs are carnivores, tadpoles can be vegetarians or omnivores. Some are filter feeders that eat algae, and others have teeth and can eat anything from rotting vegetation to other tadpoles. Either way, tadpoles tend to be voracious eaters -- it takes a lot of energy to complete their metamorphosis into frogs.