BY PROFESSOR W. A. HASWELL. 503 



ance, due to the presence of the regular!}' arranged transverse 

 bundles of muscular fibres referred to below. 



The body is enclosed in a cuticle 0-003 mm. in thickness. 

 Beneath this is a muscular layer consisting of external, longitudi- 

 nal, and internal circular fibres. The space between the body-wall 

 and the enteric canal is tilled with parenchyma, in which are 

 numerous large cells about 0*04 mm. in diameter — the cystogenic 

 cells. In young specimens these contain a coarse reticulum. 

 In mature specimens they appear more homogeneous. Their 

 ducts (processes of the cells) branch, and the branch ducts per- 

 forate the cuticle to open over the entire surface of the body. 

 There are no cystogenic cells in the tail. No rhabdites were 

 observed in any part. 



The pharynx (fig. 10) is a relatively long cylinder. The region 

 of the digestive canal following upon this behind, commonly 

 known as the oesophagus, is very short. There is nothing special 

 to be noted with regard to the structure of the various parts of 

 the digestive system. The intestine is fully developed long before 

 the Cercaria becomes mature. 



The tail (figs. 11 and 12) is a hollow cylinder with a thin wall, 

 tensely filled with a watery fluid. It has a thin cuticle, beneath 

 which is a single layer of circular muscular fibres of extreme 

 fineness. Internal to this are four bundles of longitudinal 

 muscular fibres, two dorsal and two ventral. On each side is a 

 fairly wide interval — the lateral line as it may conveniently be 

 termed — between the dorsal and the ventral bundle. Between 

 the dorsal and ventral bundles on each side run a number of 

 vertical (dorso-ventral) fibres for the most part in narrow strands 

 occasionally with nuclei. These are arranged at intervals along 

 the tail in such a way as to produce in some cases an appearance 

 of segmentation. Along each lateral line, immediately below the 

 cuticle and circular muscle, runs a delicate filament. This gives 

 off branches which run both dorsally and ventrally, frequently 

 dividing into finer threads. At several points in its course the 

 main filament presents a thickening. Accompanying this is a 

 very regular row of small, usually rounded, cells of a peculiarly 



