lander: anatomy of hemiurcs crenatus. 3 



to the posterior end of the pharynx, about midway between the two 

 suckers, or a little nearer the oral sucker. Slightly in front of the ven- 

 tral sucker is a deep, transverse, slit-like depression, which may be 

 designated as the preacetabular fossa. The width of the fossa is slightly 

 less than that of the ventral sucker. A characteristic feature of this, as 

 well as of several other species of the Hemiuridae, is the ringed appear- 

 ance of the trunk. The length of these rings varies according to the 

 state of contraction of the body, those near the posterior end of the trunk 

 being, however, several times as long as those near the anterior end. In 

 one specimen 142 of these rings were counted. The appendix is not 

 rino^ed, but usually presents one or two slight constrictions. 



Body Wall. 



The wall of the body exhibits the usual structures: viz. a cuticula, or 

 external structureless covering, a layer of circular muscle fibres, a layer of 

 longitudinal muscle fibres, and a layer of diagonal muscle fibres. 



An inner and an outer layer of the cuticula, commonly described, are 

 not distinguishable in this species. The rings noticed on the surface 

 of the body are due to variation in the thickness of the cuticula. The 

 thickness at the anterior end of a ring is about S.Qfx, whereas at 

 the posterior end of the ring it is from two to four times as great 

 (Plate 1, Fig. 5), so that when seen in profile a notch marks the posterior 

 end of each ring. 



The outer (circular) layer of muscles of the body wall consists of very 

 small fibres (about 0.5 /xin diameter) so closely set that in transverse 

 sections they appear to form a continuous sheet and in longitudinal 

 sections a row of minute dots. They are best seen in tangential sections 

 (Plate 3, Fig. 34). 



The longitudinal fibres are much larger, measuring 2 /x to 4 /a in 

 diameter. They are circular or oval in cross section, and in good prepara- 

 tions show a clear central area, surrounded by a more deeply staining 

 highly refractive portion. 



Trematode muscle fibres have been described by several authors 

 (Juel, '89 ; Pratt, "98) as having a structure similar to this. None 

 of the muscle fibres of the body wall show any traces of nuclei. 



The layer of diagonal muscle fibres is quite well developed. In trans- 

 verse sections the diagonal fibres are not easily distinguished, but their 

 size and arrangement are well shown in tangential sections (Plate 3, Fig. 

 34). They are slightly larger than the longitudinal fibres. There are 



