510 
THE COMMON FROG* — 
Il. 
BEFORE passing on to an enumeration of the subordinate 
groups of the sub-kingdom Vertebrata, we may first revert 
to our subject, the Frog, and make further acquaintance with it. 
The common frog of this country belongs to the genus Rana, 
and it is the species 7¢mforaria, therefore its scientific name is 
Rana temporaria. Ut is common in Ireland, as well as in Eng- 
land and Scotland, and is indeed the most’ widely distributed 
species of the frog-order, being found throughout the temperate 
regions of both the Old and New Worlds. It is found over 
nearly the whole of Europe ; in Africa north of the Sahara, and 
in Egypt; in Northern Asia, including Japan and Chusan, and 
it is also spread over North America. It is not found in the 
northern half of Scandinavia, nor in Iceland. j 
Except in winter, the common frog is generally in England 
so familiar an object, that any description of it might seem super- 
fluous. The purpose in view, however, renders it needful at 
least to recall certain external structural characters both of the 
adult and the immature condition. 
The head and body of the frog together forms an elongated 
oval mass, somewhat pointed at each end, of which mass the head 
constitutes rather more than one-third. This mass is more or 
less flattened both above and below, except at the commence- 
ment of the hinder third of the back, where there is a more or 
less marked prominence, which indicates the junction of the 
haunch bones with the spine. In front of this the only marked 
projections are those of the eyeballs. 
The short arms project outward on each side just behind the- 
head, and each ends in a small hand with four fingers, the 
second of which is the shortest, and the third the longest. 
When the arm is turned backwards this third finger barely at- 
tains (if it can do so at all) the hinder end of the body. 
The hind limbs proceed from quite the hinder end of the 
~body, there being no vestige of a tail. The thigh is very mus- 
cular, and the leg has a good ‘‘ calf.” The foot is exceedingly 
long, and what is very remarkabie, is so jointed that the toes 
can be sharply bent upwards on its thick and undivided part. 
The latter thus seems to form a third segment of the hind limb 
following the thigh and the leg, the limb having four segments 
instead of three as in ourselves, and in almost all beasts, birds, 
and reptiles. 
The foot ends in five toes connected by a web. Of these the 
fourth is the longest, the first the shortest. On the inner mar- 
gin of the sole of the foot, at the root of the first toe, is a small, 
hard prominence, called a ‘‘tarsaJ tubercle.” When the hind 
limb is turned forward, the knee reaches nearly to the armpit ; 
the ankle-joint is about on a line with the end of the snout, and 
both parts of the foot beyond it. These two parts of the foot 
together are much longer than the whole fore limb, and exceed 
Ay a of the length of the whole mass of the head and 
ody, 
When the animal is viewed in profile, the point of the muzzle 
is seen to be very little in advance of the opening of the mouth. 
The latter is straight. It is also very wide, extending back even 
beyond the hinder margin of the eye. Just above the hinder 
angle of the gape, and behind the eye, is a rounded surface of 
smooth, tightly-stretched skin. This is called the ‘‘ tympanum,” 
and directly covers in the drum of the ear. 
When the mouth is opened, if the finger be drawn along the 
inner margin of the upper jaw, a series of minute teeth may be 
detected. Towards the front of the palate are a pair of small 
holes (which are the inner openings of the nostrils), and between 
these are two juxtaposed little groups of other minute teeth. 
There are no teeth whatever in the lower jaw. At the hinder 
end of each side of the palate is another small hole. These 
latter two apertures are each the opening of a canal leading 
from the mouth to the cavity of the ear within the drum. The 
tongue is seen to be large, flat, and fleshy. It is tied down to 
the jaw in front, but free for more than its hinder half, with the 
processes developed from its free hinder margin. 
The skin of the frog is naked and smooth, without a trace of 
scales, or other appendages. Its colour on the upper surface is 
more or less yellowish, or reddish brown, with irregular black, 
brown, or grey patches. Similar patches form transverse bands 
upon the legs. Beneath the colour is pale yellowish, often with 
a few spots, paler than those of the back. There is constantly 
a brownish black subtriangular patch placed behind the eye, 
Continued from p. 471. 
NATURE 
and extending over the tympanum down towards the shoulder, 5 
The frog breathes partly by swallowing air (aided by a 
mechanism to be described hereafter), partly by the direct respi- 
ratory action of the skin, It feeds exclusively upon living 
animals, such as insects and slugs, which it catches by suddenly 
throwing forwards beyond the mouth, the free hinder half of the 
tongue (furnished with an adhesive secretion), and then retract- — 
ing it with its prey in a most rapid manner, 
In winter the frog passes into that torpid state known as hiber- 
nation, as is the case with our bats, hedgehogs, and some other 
beasts. Its season of torpidity is generally passed by it buried 
in mud and at the bottom of water, and great numbers of indi- 
viduals may be dug up in winter all clustered together. 
In spring the frogs again congregate for the. purpose of — 
oviposition in the month of March, at which period their well- — 
known croaking makes itself heard, and though in itself unme- 
Fic. 3.—View of left side of head of Embryo Tadpole (after Parker). 
br} and 47°, first and second external branchiz3 cé!—cé5, the six viceral 
clefts; cf, the left ‘‘holder”; d, the olfactory organ ; ¢, the eye ; df, 
th left lip ; #, the aperture of the mouth; of, the hinder margin of the 
rudimentary operculum, 
lodious, possesses a certain charm through its association with 
the vernal outburst of nature. 
When first laid, the frog’s eggs are little round dark bodies 
enclosed in no solid shell or case, but in a small glutinous enve~ 
Fic. 4.—The Edible Frog (Rana esculenta). 
lope. The latter quickly swells in the water so much that the 
“spawn” comes to have the appearance of a great mass of jelly 
through which dark specks (the yolks of the egg) are scattered, 
Each egg, when microscopically examined, may be seen to — 
undergo a process of yolk sub-division and cleavage till a mul- 
berry-like mass is formed. Upon this soon appears the ‘* pri- 
mitive groove,” which forms a canal and develops beneath it a 
“chorda dorsalis” according to the process which has been 
already stated to be common to the whole of the Vertebrata, 
Gradually the embryo assumes the form of a young tadpole, 
and is provided with a pair of little ‘‘ holders” (or organs for 
q 
[Oct. 16, 1873 
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