MUSCLES OF THE BODY- WALL. 97 



2. Muscles of the Body -wall. 



The longitudinal muscular coat (e, Plate XVIII, fig. 4), which is incorporated with the cuta- 

 neous layer {d") at its commencement, is thick and powerful, and has a well-marked fasciculated 

 aspect in transverse section. At the sides of the mouth, where it attains great develop- 

 ment, and forms a strong lateral support, there is a very pretty radiate or somewhat arborescent 

 arrangement of the interfascicular substance in transverse section (Plate XX, fig. 1). This 

 appearance is due to numerous radiating fibres, which pass from the inner longitudinal layer 

 through the circular coat, and then diverge widely in the great muscular mass (Plate XX, fig. 13). 

 It is best seen in the neighbourhood of the lateral nerve, where the muscle is most developed, and 

 especially in partially decomposed specimens. So thickly are these fibres placed, that they pass 

 through the slightly coloured stroma surrounding the nerve. Such a condition permits great 

 stretching in all directions without actual separation of the muscular bundles, and is thus emi- 

 nently adapted for the functions of the parts. The intimate connection of the outer fibres of this 

 layer with the adjoining coat is well brought out in some longitudinal sections of the body, 

 which show the outer bundles of fibres quite separated from each other by rows of pigment- and 

 other cells and granules, the whole having a curiously streaked appearance. Anteriorly this 

 longitudinal layer becomes lost in the tissues of the snout. The next coat (e) consists of a series 

 of circular muscular fibres of considerable thickness, between which and the former the nerve- 

 trunks are situated. It extends forward by the ganglia, and appears to merge into the wall of 

 the passage for the proboscis in front of these organs. Within the last-mentioned coat is a layer 

 (e") of longitudinal muscular fibres, similar in structure to the corresponding stratum in the 

 Enopla. It also passes the ganglia to become connected with the muscular channel for the pro- 

 boscis in the snout. The several muscular layers retain nearly the same relative proportions 

 towards the posterior end of the worm (Plate XVIII, fig. 11). 



The cutaneous and muscular coverings of Lineus sanguineus are thinner than in Z. gesserensis, 

 but conform exactly to the same type. The only peculiarities observed in the muscular coats of 

 Lineus marinus are the very evident transverse streaks of the external longitudinal (Plate 

 XVIII, fig. 6, e), and the presence of certain cellular masses in it and the next outer layer. 

 These bodies (Plate XXII, fig. 5, a) lie in definite spaces (6), and consist of groups of rounded 

 cells filled with granules. In the contracted state of the animal, as after preservation in spirit, 

 the circular coat in longitudinal sections has a wavy aspect (<?', Plate XVIII, fig. 6), 

 apparently from the extreme shortening of the parts. In L. bilineatus the circular muscular 

 layer is thicker than in L. gesserensis, a condition probably connected with the somewhat rounded 

 form of the worm. The enormous muscular layers in Borlasia Elizabeths (Plate XXI, fig. 4) 

 have a fine red hue, so that the resemblance in this respect to the muscles of the higher 

 animals is striking. The reddish coloration is most intense on each side of the circular coat all 

 round, especially at the region of the nerve-cords, which are paler than their investments. The 

 circular muscular coat is less tinted than the others, and forms a distinct line of separation between 

 them. In this species, also, the fixing of the longitudinal fibres by the interfascicular substance is 

 very favourably seen. In certain Lineidce from Shetland the inner longitudinal muscular layer 



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