THE FROG 581 
nate—buried in the mud of the ditches and ponds, mouth 
shut, nose shut, eyes shut—and breathe through their skin. 
Form and external features.—The absence of neck and 
tail, the short fore-limbs almost without thumbs, the longer 
hind-limbs with five webbed nailless toes and with a long 
ankle region, the apparent hump-back where the hip-girdle 
is linked to the vertebral column. There is a very rudi- 
mentary thumb, and there is a horny knob at the base of 
the hallux or “great toe.” At pairing time the skin of the 
first finger is modified in the males into a rough cushion, 
darkly coloured in 2. temporaria. 
The wide mouth, the valvular nostrils, the protruding 
eyes, the upper eyelid thick, pigmented, and slightly mov- 
able, the lower rudimentary and immovable, the third 
eyelid or nictitating membrane semi-transparent and moving 
very freely, the circular drum of the ear, the slightly dorsal 
cloacal aperture. 
Skin.—The smooth, moist skin is loosely attached at 
intervals to the muscles by bands of connective tissue, 
which form the boundaries of over a score of lymph-sacs. 
These contain fluid partly absorbed through the skin, and 
open into the veins by two pairs of lymph-hearts. The skin 
consists of a two-layered (ectodermic) epidermis, and an 
internal (mesodermic) dermis. The transparent outer layer 
of the epidermis is shed periodically, and swallowed by the 
frog. The dermis differs markedly from that of a fish, for- 
there is no exoskeleton, though this was present in the 
extinct Labyrinthodonts; there are multicellular glands, 
whose secretion keeps the skin moist and is in part 
poisonous; and there is a stratum of unstriped muscle 
fibres. Pigment cells occur in the dermis, and’ some 
extend between the cells of the epidermis. The colour 
changes a little according to the state of these cells, the 
protoplasm expanding and contracting partly through the 
direct influence of light and moisture on the skin, partly by 
a more complex reflex action in which the eyes, the brain, 
and the sympathetic nervous system are all implicated. In 
the larval salamander the pigment cell seems to contract 
and expand as a whole, but this is not usually the case. 
There are cutaneous blood vessels, by means of which the 
frog can, to a certain extent, breathe by its skin. The 
