THE ANATOMY OP PENTASTOMUM TEEETIUSCULUM. 11 



So far as I have been able to observe, all the muscles of 

 the body are distinctly striated. 



Circularly disposed Fibres {M. Tr.). — These are pre- 

 sent all over the body, save in the head region, are placed just 

 within the layer of cuticle-secreting cells, and pass right across 

 the dorsal and ventral surfaces (figs. 12, &c., 29). The layer 

 thins out along two lines which correspond in position to the 

 part of the body-wall which lies midway between the dorsal 

 and ventral insertions of the oblique muscles (figs. 12 — 23, L.). 

 It is this absence of the circular layer of fibres which results 

 in the appearance of a dark line — the lateral line — running 

 along each side of the body. Elsewhere the fibres give the 

 body-wall a dull, whitish, opaque appearance, but here there 

 is little between the cuticle and the body-cavity. The latter 

 being a closed-in and non-illumined space, and the cuticle 

 through which it is seen fairly transparent, the appearance of 

 a dark line is of necessity produced. The circular layer of 

 fibres is a thin one, and frequently, but not always, only one 

 fibre thick. The fibres are apparently hollow, and are often 

 broken up into fibrils of irregular size, perhaps owing to the 

 action of reagents. Sometimes the fibre appears to be formed 

 of a little group of these fibrils (fig. 51, M. Tr.). The layer 

 seems to stop short just beyond the first annulation, as, in 

 longitudinal sections, no circularly disposed fibres are seen 

 in the head region (fig. 24). 



Longitudinally disposed Fibres (M. Long.). — With 

 the exception of the ventral surface of the head, where the 

 hooks arise, these fibres form a layer over the whole surface 

 of the body just within the layer of circularly disposed fibres. 

 In P. protelis the two layers are separated by a considerable 

 interval, corresponding in extent to almost the whole thick- 

 ness of the body-wall, the space being occupied by a large 

 number of cells absent in P. teretiusculum. The fibres 

 composing this layer difi'er from those of the first mentioned, 

 inasmuch as they are not hollow, and show no sign of breaking 

 up, when cut transversely, into groups of fibrils, such as are 

 met with in the circularly disposed fibres (figs. 29, 51, 



