Pigment of the enderon. — The enderon presents scattered rnasses of pigment, some times contained in cells and sometimes free, in many Invertebrata (Annelids, Trematoda, Echinoderms, Crustacea, Mollusca). In other Invertebrata and in the higher Verte brata, the pigment is confined to the ecderon. In Fishes and Reptiles, however, a well marked layer of pigment lies at the surface of the enderon in the form of scattered granules and of irregular more or less stellate masses hich are not enclosed in cells. The silvery lustre of the skin of fishes is due to minute rods which constitute a layer at this surface, and should probably be regarded as a peculiar form of pigment granules.
In the Cephalopoda and some Gasteropoda among the Invertebrata, the integument undergoes during life the most extraordinary variations of colour, becoming overspread with successive clouds of the most vivid hues. These are produced by the contraction and expansion of peculiar sacs — the chromato phora—containing masses of pigment granules. According to H. Miiller, (whose observations I have receutly had the opportunity of re peating,) these are sacs attached to whose walls are contractile fibre cells arranged ra dially, and frequently anastomosing with those of other cells. They do not always contain pigment, but frequently present a distinct nu cleus. Several layers of these chromatophora of different colours are frequently disposed, one over the other, in a given portion of the skin, and produce by their different states of contraction, relatively to one another, suc cessive changes in the colour of the spot.
Among the Vertebrata tbe Chammleon, as is well known, presents similar phenomena.
Papilke of the enderon.— The enderon is frequently produced into conical or cylin drical processes, which either merely contain a vascular loop, or are supplied, in addition, with special nerves. In the Invertebrate, we find, in the processes of the mantle into the shell of the Brachiopoda described by Dr. Carpenter, organs which, I have no doubt, must be reg,-,arded, like the corresponding pro cesses in the Ascidians, as vascular papillm. Among the Articulata like processes extend, in the Crustacea, through the whole thickness of the integument to its surface, giving rise to the colourless spots observable on the shell of the crab, for instance. I irnagine, however, that these spots were usually occupied by a hair when the shell was thin. In the Mollusea, the marginal processes of the mantle of the Lamellibranchs and Gasteropods, the papillm of Onchidium, &c. and those of Tremoctopus
(H. Milner) are very probably both vascular. and nervous papillm like those of fishes.
Among the Vertebrate, fishes present large projecting papillm, particularly about the region of the lips and operculum, which are both vas cular and nervous. Simple papillx (nervous?) are scattered over the surface of the body in Plag,iostomes and some Ganoid fishes.
I am not aware that papillae have hitherto been observed on the integument of Birds and Reptiles. In most Mammals, they are very small, if they exist at all, upon the general sur face of the body, attaining a considerable size only in such organs as the ball of the foot (Cat, Dog), or on the muzzle. The Cetacea, however, appear to make a remarkable excep tion to this rule ; it is stated (Heusinger, Breschet, and Roussel de Vauzeme) that the very thick integument of these animals is tra versed by vascular and nervous papillm, four or five lines long, which extend as far as the outer horizontal horny layer of the ecderon, so that a horizontal section of the ecderon is like that of a horse's hoof. In man, again, the papillm are, as is well known, so abundant as to have given rise to the term pars papillaris, for the superficial layer of the ecderon. The structure of those which appear to possess special nervous functions will be considered below.
Sensory appendages of the enderan.— Very little is known of the ultimate distribution of the nerves to the integument in the Inverte brate, but we are indebted to Leydig for showing that in certain Crustacea, Insecta, and Mollusca, it is very similar to what occurs in the vertebrate classes. Thus in Arplus foliaceus the peripheral nerves become pale, and divide, and at the point of division there is a nucleus ' as in the embryonic fibres of the frog. In Artenda salina, Branchipus stagnalis, and in the Heteropod Mollusk Carinaria, the termination of the tegumentary nerves is es sentially similar. The larva of the Dipterous insect Coretbra, presents even peculiar sensory appendages, in the delicate plumed hairs I.vhich beset the sides of the body. These are articulated in the ordinary way, and have an internal ligament, a sort of spring, attached to their base, which is enlarged and receives the enlarged and cellmform termination of a nervous twig. It will be obvious that this arrangement is peculiarly fitted for commu nicating the slightest vibration to the nerves.