b. C. α. a. great number of minute square areas or facets, each of which shews faint signs of furrows crossing it diagonally from corner to corner. Imbed the eye and cut a number of sections from it perpendicular to its surface: mount in glycerine and examine with one inch objective. If the section has passed through the middle of the eye it will be seen to present a central mass (optic ganglion) from which a number of lines appear to radiate to the facets on the surface. These radiating lines (which are obscured here and there by pigmented layers) are indications of the striated spindles, connective rods and crystalline cones. Examine your thinnest section with a high power, or tease out one of your thicker ones in glycerine. Beginning at the exterior make out successively The cornea, answering to one of the superficial facets. Its flat outer and slightly convex inner surface. Immediately beneath the cornea there will be seen (in good specimens) a slightly granular layer. B. The crystalline cone, an angular transparent body which is usually obscured by pigment. If this is the case, another section must be mounted in dilute caustic potash, which removes the pigment. Behind the crystalline cone comes the connective rod. It is widest in front where it joins the cone but narrows posteriorly continuous with the striated where it is 8. E. ند spindle. If fresh eyes be treated with osmic acid and then teased out, each of these rods can be split up into four fibres. The striated body is fusiform and presents well-marked transverse striations. Besides these coarse striations, however, much finer ones can be seen by careful examination with a high power. The outer ends of these spindles correspond in position to the second of the pigmented layers seen with the low power (b. a.): they are best seen in specimens treated with dilute caustic potash. Beneath the striated spindles is a perforated membrane through which the spindles pass to become continuous with the optic ganglion. From their ends pass nerve-fibres which run inwards in a converging manner and among which nerve-cells are here and there scattered. Within the ganglion are several concentric pigmented bands. If the section has passed back along the optic nerve two obliquely placed lenticular masses will be seen among its fibres. ໗. Passing back from the cornea to the optic ganglion is a membrane investing each cone, rod, and spindle. It is on this that most of the pigment lies which causes the two outer dark bands. Over the rods the pigment is wanting and there the membrane is seen to possess oval nuclei. 24. The Auditory organ. This lies in the basal joint of the antennule and is best examined in the lobster. The upper surface of this basal joint is flat posteriorly and joins in front at an angle a rounded anterior portion. It bears several tufts of hairs: one of these is very small and lies at the inner side of the flattened surface, just at the angle where it meets the rounded part; among these hairs is the opening into the auditory sac, through which a bristle can easily be passed. a. b. Take a fresh antennule from a lobster and cut If this sac be held up to the light a little patch α. The white line will be seen to answer to a ridge on the apex of which is a row of large hairs, and both on the brown patch and on the opposite side of the main row will be seen scattered groups of smaller hairs. C. α. Examine with obj. Each of the hairs seen with the lower power is now seen to be covered over its whole surface with innumerable very fine secondary hairs; these are shortest near the base of the primary hair. Towards its base each of the primary hairs is constricted and then dilates into a bulbous enlargement which is fixed to the wall of the sac. B. The brown patch is seen to owe its colour to a single layer of polygonal epithelial cells containing pigment granules. By focussing through this epithelial layer a number of parallel slightly granular bands is seen passing up, one to the base of each hair in the main row on the top of the ridge. At the base of the hair to which it runs, each band is constricted and, entering the bulbous enlargement of the hair, joins a small hemispherical swelling within it. If a fresh auditory sac be put in 1 per cent. solution of osmic acid for half an hour, and then laid for twenty-four hours in distilled water and examined, each of the granular bands mentioned above is seen to consist of a bundle of fine fibres which swell out into fusiform enlargements at intervals. A great part of the whole interior of the auditory sac of the lobster is covered with very fine hairs which can only be seen with a high power. Epithelium is absent except the pigmented patch above mentioned. 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, |