Annelides. 19 



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 ma- 

 ceration 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 ar- 

 ranged 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 beau- 

 tiful, but in the species under consideration, from there being little or 

 no variation from two colours, the arrangement is comparatively sim- 

 ple. 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 fi- 

 brous 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 de- 

 termined 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 ar- 

 ranged in a spiral form around the body, each stratum taking an 

 opposite direction to that of the other, so that by the continual decus- 

 sation of the fibres of each stratum taking place a diamond-like ap- 

 pearance is produced, (fig. 6-). 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 fascicuU 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 ordi- 

 nary 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 cu'cular layer is the 

 most conspicuous, and is best developed on the outer margin of each 



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