Cutaneous system. The cutaneous investment may be divided into three layers,—the epidermis, the pigmental layer, and the dermis or true skin. The epidermis is soft and easily lacerable; it is so thin that it cannot well be dissected away from the pigment, but by maceration in turpentine or weak acid it will easily separate from the other layers; it is periodically shed, like that of the frog and snake, and consists principally of tesselated epithelium. The pigmental layer consists of a series of minute cells of an oval figure, containing colouring matter, which on the dorsal surface is of a dark green, but on the ventral of a yellow colour; in some parts the cells are arranged in a series of diamond- shaped figures, as if they followed the course of the blood-vessels or the oblique layer of muscular fibres: in the medicinal leech the arrangement of the pigment is very beautiful, but in the species under consideration, from there being little or no variation from two colours, the arrangement is comparatively simple. The true skin is so intimately blended with the muscular tunic as to be very difficult of examination; it is freely supplied with nerves and blood-vessels, is of a reddish colour, and appears to be of a fibrous nature.
The muscular tunic itself, upon a careful examination, is found to be composed of three strata or layers of fibres, running in different directions: the layer nearest or immediately underneath the cutis is composed of a series of circular fibres; these are arranged in bundles, quite distinct from one another, and the breadth of the rings is determined by the number of fibres in each bundle, (fig. b); it requires very great care to see these fibres distinctly, as they are in general removed with the cutis.
The next layer is composed of two strata of fibres, which are arranged in a spiral form around the body, each stratum taking an opposite direction to that of the other, so that by the continual decussation of the fibres of each stratum taking place a diamond-like appearance is produced, (fig. c). The innermost layer of all is composed of fibres, arranged also in fasciculi; these take a longitudinal course from the head to the tail of the animal, (fig. d); the fibres composing these fasciculi are much larger, and the layer itself more developed, than either of the two preceding, which can readily be accounted for when we consider that these are the fibres which are most used in the ordinary locomotion of the animal. At the head and tail the whole of the three layers of muscular fibres are at first slightly constricted, and they then expand to form the sucking discs; the circular layer is the most conspicuous, and is best developed on the outer margin of each
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