The limbs are peculiar. Not only are they relatively long and very slender, but two digits are permanently opposed to the other three. On the hand the first three fingers form an inner bundle opposed to the outer, or fourth and fifth fingers. On the foot the inner bundle is formed by the first and second, the outer by the other toes. The shoulder-girdle is of the ordinary Saurian type, but there are no clavicles and no interclavicle. The costal sternum is well developed; the ribs posterior to those which meet the sternum are very thin and elongated: they meet and fuse with their fellows in the medio-ventral line. These hoops are not connected with their neighbours in front or behind. The tail is prehensile by being rolled downwards; it is not brittle and is incapable of being renewed. The skin is not covered with scales, but with granules. The eyes are very remarkable. The eyeballs themselves are large, but the eyelids are united into one fold with a small central opening. However, when the Chameleon is asleep the margins of this opening sometimes become more slit-like. The right and left eye can be, and are incessantly, moved separately from each other, and the creature squints terribly. Each eyeball, together with the pin-hole eyelid, is rolled up and down, backwards and forwards, independently of the other eye. This is a unique feature, but it also occurs in people who squint badly. The question "What, and how, do these creatures see?" is therefore quite idle, especially since in reptiles binocular vision does not exist at all and, consequently, cannot be disturbed by squinting.
The tongue has attained an extraordinary development. The tongue proper (Fig. 152) is club-shaped, and is covered with a sticky secretion. The base or root of the tongue is very narrow, composed of extremely elastic fibres, and is supported by a much-elongated copular piece of the hyoid. The elastic part of the tongue is, so to speak, telescoped over the style-shaped copula, and the whole apparatus is kept in a contracted state like a spring in a tube.
Fig. 149.–A, Dorsal, B, ventral, and C, lateral view of the skull of Chamaeleon vulgaris. × 1. Cond, occipital condyle; EP, ectopterygoid; Jug, jugal; Lac, lacrymal; Pal, palatine; Par, parietal; Prf, prefrontal; Pt.f, postfrontal; Ptg, pterygoid; Q, quadrate; Sq, squamosal; Vo, vomer.
A pair of wide, very elastic blood-vessels and special elastic bands extend from the base into the thick end of the tongue. By rapidly filling the apparatus with blood, and by the action of certain hyoid muscles, the spring is, so to speak, released, and the momentum gained by the thick and heavy club-shaped tongue proper projects it far out of the mouth. The sticky end of the club shapes itself into an upper and a lower flap, which partly envelop the prey, and the elastic bands of the far-stretched stalk withdraw the whole. The detailed working of this ingenious shooting apparatus is not easy to follow. An ordinary full-grown Chameleon can shoot a fly at the distance of 7 or 8 inches. The whole performance is very quick, lasting less than one second. When the desired object is very near, only 2 or 3 inches off, the Chameleon has a certain difficulty in shooting its prey. The tongue is at first put out slowly, tentatively, the following jerk is feeble, and it seems as if the apparatus refuses to work unless it is allowed to shoot out with full force.
Another remarkable and quite proverbial feature of Chameleons is their changing of colour. This is by no means restricted to Chameleons, which indeed are rivalled in this respect by various other lizards, for instance by the Indian Agamoid Calotes and by the American Ameiva.
The microscopical structure and mechanism of the colour-changing apparatus is, in Chamaeleon vulgaris, as follows:–
The epidermis is colourless, and the Malpighian layer is not particularly modified except that in it are imbedded some iridescent cells, with very minute wavy striation on their surface. The cutis contains in its leathery tissue a great number of small and closely packed cells, filled with strongly refractive granules, chiefly guanine-crystals. These cause the white colour by diffuse reflection of direct light. The cells nearer the surface are charged with oil-drops and appear yellow. Large chromatophores are imbedded in the white granular mass, most of them with blackish-brown, others with reddish pigment, the granules of which are shifted up and down, towards and away from the surface of the cutis, in ramified branches of the chromatophores. When these branches are contracted the pigment is conveyed back into the bulbous basal portion of the chromatophores and the skin appears yellow or white. When all the pigment is shifted towards the surface of the cutis, the animal looks dark, sometimes black. In intermediate conditions the light is changed into green by diffraction through the yellowish upper strata and by the finely striated iridescent cells of the Malpighian layer. Those parts into which the chromatophores do not send pigment appear as yellow spots. The chromatophores are to a great extent under control of the will of the Chameleon, but external stimuli, as heat and cold and other reflex actions, also play a great part in their movements.
For further information on this subject see Brücke,[[168]] P. Bert,[[169]] Pouchet,[[170]] Thilenius,[[171]] and lastly Keller,[[172]] who has written a very long but rather confused account.
The process of moulting is curious. When the Chameleon is in good health the whole process is accomplished within a few hours. The skin to be cast off becomes loose and assumes a blistered appearance. Sometimes the creature looks as if it were wrapped up in white, semi-transparent tissue paper. By rubbing against stones, or between the twigs of trees, the skin comes off in large flakes, first on the lips, then on the contorted body, and last on the under surface of the hands and feet. During a rapid and successful moult the changes of colour go on as usual in the new skin. Sometimes large flakes of the old skin remain adherent for days, especially on the top of the head. The moulting takes place several times in one year. One of my Ch. vulgaris moulted in January and September, and then not until June of the next following year. A Ch. pumilus moulted in the months of May, October, and March.