12 W. BALDWIN SPENCER. 



M. Long.). The structure of the muscle fibres is shown in 

 fig. 30. They are seen to have a very characteristic appear- 

 ance : at regular intervals the customary dark lines pass across 

 the fibre, a light band is present on either side of the dark 

 line, and between two consecutive lines there can be seen, on 

 examination with a high power, what have the appearance of 

 minute rod-like structures lying parallel to each other and to 

 the length of the fibre. Whilst there is no definite sarcolemraa 

 present the fibres are bound together by a material, usually 

 of a slightly punctated nature, in which very distinct nuclei 

 are present (Nu.). But rarely the outline of cells can be dis- 

 tinguished in this layer, which agrees closely with that 

 described by Leuckart as present in many cases — as, for ex- 

 ample, P. proboscideum. This layer of muscles is thicker 

 than the circular one, and passes completely round the body, 

 there being no definite arrangement of the fibres into bundles 

 (figs. 13, &c., 24). 



Obliquely disposed Fibres [O.m.). — These run obliquely 

 across the body-cavity, their dorsal ends being inserted into 

 the body-wall at a level nearer to the mid-dorsal than to the 

 mid-ventral line, and their ventral ends being inserted on either 

 side of and not far from the mid-ventral line (figs. 12, &c., 28, 

 0. m.). They serve roughly to divide the body-cavity into three 

 compartments: (1) a median one stretching from the dorsal to 

 the ventral surface, and containing the alimentary canal, repro- 

 ductive organs, nervous system, hook-glands, &c. ; and (2) 

 two lateral ones, one on either side (L. S.). A transverse 

 section of the body thus roughly resembles, in this division of 

 the coclom into three parts, one through the middle of the 

 segment of such a polychsete worm, for example, as a Poly- 

 ophthalmus, though of course no structures comparable to 

 parapodia are present. The oblique fibres branch and split up 

 into fibrils at either end, and these fibrils pass through between 

 the muscle layers, some apparently passing into the layer of 

 circular fibres, others running through the cuticle (fig. 28). 



In young forms especially, a division of these fibres into 

 two groups on each side of the annulus can be clearly seen 



