544 H. LYSTER JAMESON, 



preparations (Fig. 9 s), 2) a cortical layer or "Rinden schiebt" (Fig. 7), 

 composed of the so called "contractile plates", staining deeply and pre- 

 senting the well known radial arrangement, and 3) a central plasma, the 

 "kerntragende Plasmatheil" (Hesse, 7, p. 46) lying sometimes completely 

 enclosed by the contractile elements, sometimes projecting at one point 

 and bearing an eccentrically lying nucleus (Figs. 7 and 8). I believe the 

 sections which show the plasma completely surrounded by the contractile 

 elements are sections of fibres at some distance from their centre, and 

 that it is probably only in the region of the nucleus that the proto- 

 plasm forms a marked projection. This one sees more plainly in 

 macerated preparations (Figs. 9 and 10). The same is also the case 

 in Ascaris, where however the open part of the fibre is relatively 

 much larger. 



The nucleus is almost always, if not universally, peripheral in 

 position as was found also by Spengel in Echiurus (17, tab. 24, 

 fig. 24). The cortex is often so highly developed as to leave but a 

 very small plasmatic space in the axis, and often it appears to ob- 

 literate the axial plasma altogether towards the ends of the fibres- 

 Fig. 8 a, b, c, d show progressive stages of this obliteration. The 

 fibres are never flattened into plate-like structures as in some Poly- 

 chaets. 



In longitudinal sections, and still better in macerated preparations, 

 the fibres show a longitudinal striation. The striae which correspond 

 to the "contractile elements" already seen in transverse sections are 

 sometimes parallel to the long axis of the fibre (Fig. 9), sometimes 

 spirally arranged around it (Fig. 10). 



In such spirally striated fibres one can also see the striae belong- 

 ing to the two sides of the fibre at the same time, which might easily 

 lead to a conception of an intercrossing system of diagonal lines. 

 Careful focussing however makes the real nature of this appearance 

 obvious. The fibres taper off" very gradually towards both ends. In 

 a few small fibres seen in transverse section of the proboscis, which 

 may be young muscle cells, the nucleus occupied a somewhat more 

 central position (Fig. 8e). 



The outermost hyaline layer or sarcolemma is bounded out- 

 wardly by a fine refractive membrane, comparable to the pellicula 

 described by Bütschli in the muscle cells of Ascaris (2). In the 

 sarcolemma I could not detect any structure. To the pellicula are 

 attached the little languets referred to above, which probably bind 

 the fibre to the surrounding tissues. 



