162 ILLINOIS BIOLOGICAL MONOGRAPHS [450 



tip of the scolex to assist in activating that region. Between the bothria, how- 

 ever, they were found to be separated into dorsal and ventral layers as in the 

 strobila, and not united into a single coronal band, as shown by Johnstone. 

 The bothrial sphincter (Fig. 48) is a powerful bundle of fibres, about 0.07mra. 

 in transverse section surrounding the aperture close to its cuticula. In trans- 

 verse sections of the scolex it appears as a deeply staining mass on each side 

 of the opening, also shown in Johnstone's figure 15. As it crosses the aperture 

 anteriorly it becomes greatly attenuated. This with its comparatively great 

 size at the sides and posteriorly accounts for the almost complete disappearance 

 of the aperture in many adult, preserved scolices owing to the powerful con- 

 traction of this muscle from behind forward, thus diminishing the opening 

 towards the tip of the scolex. From their arrangement it is to be seen that this 

 sphincter, evidently a modified group of frontal fibres, and the longitudinal 

 muscles in the scolex play a more important role in the movements of the 

 bothria than do the other groups. On account of their obHque course the 

 longitudinal fibres evidently act in diminishing the size of the lumen of the 

 bothrium as well as do the circular frontal fibres of the latter. 



The nervous system consists of two longitudinal strands which enlarge in 

 tip of the scolex to form two somewhat elongated gangHa. The latter are united 

 by only a few fibres, but they send out comparatively large nerves to the 

 bothria. In the strobila the chief strands, each from 15 to 20/i in diameter, are 

 situated ventrally in the medullary parenchyma, just within the longitudinal 

 muscles and from one-fifth to one-quarter the width of the strobila from its 

 lateral margins (Fig. 90). About halfway along the scolex the strands are 

 about 80ju in diameter; but the ganglia are somewhat smaller and situated close 

 together about 0.15mm. from the summit. In other words the chief strands 

 enlarge and diverge gradually until the equatorial region of the scolex is reached 

 and then diminish in size as they converge to form the ganglia. A pair of 

 prominent nerves is sent forward on each side to supply the saddle-shaped 

 groove described above. In young strobilas the nerve strands are situated 

 midway between the dorsal and ventral surfaces, and not ventrally. 



The excretory system consists of a pair of longitudinal vessels, situated 

 ventrally, that is, in the same frontal plane as the chief nerve strands, each 

 vessel being in the anterior end of the strobila about halfway between the 

 nerve strand and the median row of reproductive rudiments. These vessels 

 break up in a very irregular manner into extremely elongated loops, so that for 

 considerable stretches four vessels will appear while again the branchings will 

 be so numerous as to make it very difficult to decide, on looking at a transverse 

 section, which are the main channels (Fig. 48). In other individuals four ves- 

 sels appear, so that one is led to conclude that the pair just mentioned represent 

 the latter, fused at times but separated again to form the loops. But whether 

 these four vessels represent the typical four of other orders is a matter of con- 

 jecture. These main vessels may continue back into the ripe joints close 

 alongside the uterus-sacs, but they usually break up into a very diffuse reticu- 

 limi throughout the medullary parenchyma in the region where the openings 



