d. The auditory sac in the crayfish is very similar to that in the lobster, and may be examined in a similar way. It is however not so good, both on account of its smaller size and because the auditory hairs, although longer, are collected in a close tuft, which makes it more difficult to see the manner of their insertion.

XIII.

THE FROG (Rana temporaria and Rana esculenta).

THE only species of Frog indigenous in Britain is that.termed the 'common' or 'Grass Frog' (Rana temporaria), while, on the Continent, there is, in addition to this, another no less abundant species, the hind-limbs of which are considered a delicacy, whence it has received the name of the 'Edible Frog' (Rana esculenta). Unless the contrary be expressly stated, the description here given applies to both species. The Edible Frog is usually larger than the other, and is therefore more convenient for most anatomical and physiological purposes.

In the body of the Frog the head and trunk are readily distinguishable; but there is no tail and no neck, the contours of the head passing gradually into those of the body, and the fore-limbs being situated immediately behind the head. There are two pairs of limbs, one anterior and one posterior. The whole body is invested by a smooth moist integument, on which neither hairs, scales, nor other forms of exoskeleton are visible; but hard parts, which constitute the endoskeleton, may readily be felt through the integument in the head, trunk and limbs.

The yellowish ground-colour of the skin is diversified by patches of a more or less intense black, brown, greenish, or reddish-yellow colour, and, in the Grass Frog, there is a large, deep brown or black patch on each side of the head,

behind the eyes, which is very characteristic of the species. The coloration of different frogs of the same species differs widely; and the same frog will be found to change its colour, becoming dark in a dark place, and light if exposed to the light.

The body of the Frog presents only two median apertures, the wide mouth and the small cloacal aperture. The latter is situated at the posterior end of the body, but rather on its upper side than at its actual termination. It is commonly termed the anus, but it must be recollected that it does not exactly correspond with the aperture so termed in the Mammalia.

The two nostrils, or external nares, are seen at some distance from one another upon the dorsal aspect of the head, between the eyes and its anterior contour. The eyes are large and projecting, with well-developed lids, which shut over them when they are retracted; and, behind the eye, on each side of the head, there is a broad circular area of integument, somewhat different in colour and texture from that which surrounds it; this is the outer layer of the membrane of the tympanum, or drum of the ear.

The fore-legs are very much shorter than the hind-legs. Each fore-limb is divided into a brachium, antebrachium and manus, which correspond with the arm, fore-arm and hand in Man. The manus possesses four visible digits which answer to the second, third, fourth, and fifth fingers in Man. There is no web between the digits of the manus.

The hind-legs are similarly marked out into three divisions, femur, crus, and pes, of which the femur answers to the thigh, the crus to the leg, and the pes to the foot, in Man. The pes is remarkable not only for its great relative size as a whole, but for the elongation of the region which answers to the tarsus in Man. It will be observed, however,

that there is no projecting heel. There are five long and slender digits, which correspond with the five toes in Man, and are united together by thin extensions of the integument constituting the web. The innermost and shortest answers to the hallux, or great toe, in Man.

At the base of the hallux, the integument of the sole presents a small horny prominence, and sometimes there is a similar but smaller elevation on the outer side of the foot: but there are no nails upon the ends of any of the digits of either the pes or the manus. Thickenings, or callosities, of the integument, however, occur beneath the joints of the digits, both in the pes and the manus.

During the breeding season, the integument on the palmar surface of the innermost digit of the manus, in the male, becomes converted into a rough and swollen cushion, which, in the Grass Frog, acquires a dark-brown or black colour.

The Frog, when at rest, habitually assumes a sitting posture much like that of a dog or cat. Under these circumstances the back appears humped, the posterior half being inclined at a sharp angle with the anterior half. The vertebral column, however, will be found to be straight, and the apparent hump-back arises, not from any bend in the vertebral column, but from the manner in which the long iliac bones are set on to the sacrum.

The walk of the Frog is slow and awkward, but it leaps with great force, by the sudden extension of the hind-limbs, and it is an admirable swimmer.

In a living Frog, the nostrils will be seen to be alternately opened and shut, while the integument covering the under side of the throat is swollen out and flattened. The alternate pumping in and expulsion of the air needed for the Frog's respiration is connected with these movements.

The upper eyelid of the Frog is large and covered with ordinary pigmented integument, and it has very little mobility. What performs the function of the lower eyelid in Man, is a fold of the integument of which very little is pigmented and which is, for the most part, semi-transparent, so as to resemble the nictitating membrane of a bird rather than an ordinary lower lid. If the surface of the cornea be touched, the eyeball is drawn inwards under the upper lid, which descends a little, at the same time as the lower lid ascends over the ball, to meet the upper lid and close the eye.

As is well known, Frogs emit a peculiar croaking sound, their vocal powers being more especially manifested in the breeding season, when they collect together at the surface of ponds, pools and sluggish streams, in great numbers. At this season, which commences in the early spring for the Grass Frog, but much later on in the year for the Edible Frog, the male seeks the female and, clasping her body tightly with his fore-limbs, remains in this position for days or even weeks, until her ova are discharged, when he fecundates them by a simultaneous out-pouring of the seminal fluid. Shortly after the eggs pass into the water, the thin layer of viscid albumen, secreted by the oviduct, with which each egg is surrounded, swells up by imbibition and, with that which surrounds the others, it gives rise to a gelatinous mass in which the eggs remain imbedded during the early stages of their development.

The development of the eggs is closely dependent upon temperature, being greatly accelerated by warmth and retarded by cold. The process of yelk-division, which commences within a few hours of impregnation, is readily observed when the eggs are examined as opaque objects under a low power of the microscope.

While still within the egg the embryo assumes the form