427 J PSE UDOPH YLLIDEA FROM FISHES— COOPER 1 39 



mature proglottides the layer of frontal fibres just external to the main longi- 

 tudinal group is greatly thickened close to the posterior border of the segment. 

 There it forms a transverse ropehke strand, no doubt owing in part at least to 

 the numerous vitelline follicles situated in the cortex between these levels. 

 And this statement applies in like manner, but especially more towards the 

 median Hne of the strobila, to the innermost series of frontal muscles. These, 

 however, are further interfered with chiefly by the uteri and cirrus-sacs. 



The sagittal or dorsoventral fibres are quite prominent on account of the 

 fact that they are arranged in bundles which in the anterior segments find their 

 way from the medulla out into the cortex between the fasicles of the main 

 longitudinal series. They are less numerous laterally than medially. In 

 mature segments, on the other hand, they are crowded and somewhat oblit- 

 erated medially by the large genital organs, and are consequently more num- 

 erous laterally, that is, beyond the edges of the uterus-sacs. Longitudinal 

 sections show that large nimibers of them pass to the tip of the salient posterior 

 borders of the segments and hence serve to retract the latter. 



The longitudinal muscles are present in two groups, an inner, or main, 

 and an outer series. The former appears as a very conspicuous layer of large 

 fibres arranged in fasicles or bundles on each surface of the strobila and occupy- 

 ing one-half or more of the dorsoventral diameter of the cortical parenchyma. 

 The latter are on the average much smaller, less numerous, more dispersed, and 

 situated in the portion of the cortex between the outer frontal fibres and the 

 cuticula. In the anterior segments the layer of inner fibres is about lOfx in 

 thickness in the median line, and thins out gradually laterally where it joins 

 its fellow of the opposite surface by a few fibres which lie in the plane of the 

 flat, ribbon-hke medulla (Fig. 87). Linton does not seem to have noticed this 

 lateral union of the two layers, since he said that "It [the longitudinal muscle 

 layer] is, moreover, interrupted at the margins where it is penetrated by the 

 margins of the inner core [the medulla]." Farther back where the segments 

 become very broad and short it averages about 85/x in thickness, while in mature 

 proglottides the thickness amounts to 145/i. At the same time the fasicles, 

 in the anterior segments quite rectangular in outline, the longer diameter 

 being dorsoventral in direction, become more elliptical in transection. 

 In the latter case the individual fibres are circular to elliptical in trans- 

 verse section and have a maximum diameter of 15^. In describing this group 

 of longitudinal muscles in "transverse sections made thru that part of the 

 body which is immediately in front of the segments that contain ripe ova," 

 Linton stated that they "... are very large, altho not at this point in distinct 

 fasciculi." This was not found to be the case in the sections studied by the 

 writer, since fascicuH were seen all along the strobila even in the base of the 

 scolex, altho it is true that anteriorly they are separated by only a small amount 

 of parenchyma besides the bundles of sagittal fibres passing between them. As 

 regards the other series of longitudinal fibres Linton rightly observed that "the 

 longitudinal fibres of the inner part of the granular layer [here described as the 

 outer portion of the cortex] do not differ essentially from those of the longi- 



