lander: anatomy of hemiueus crenatus. 9 



to be undifFerentiated embryonic cells persisting in the adult. It seems 

 probable that, up to a late stage in the life of the worm, they continue to 

 give rise to cells which become differentiated into muscular elements and 

 possibly into other tissues. 



A striking feature of the parenchyma in this species is the large cells 

 which form a sheath around the two intestinal coeca from the region of 

 the vitellaria to their posterior ends (Plate 2, Fig, 2o, pa'ench. cL). The 

 protoplasm of these cells is distinctly granular, and each shows a clear 

 spherical or oval nucleus provided with a single chromatic granule. They 

 are usually arranged in a single layer around each of the crura ; occa- 

 sional cells occur in the parenchyma anterior to the point where the 

 sheath ceases. I have been unable to find in any descriptions of the 

 parenchyma of trematodes an equally extensive tissue which has retained 

 so perfectly its cellular condition. The parenchyma of the posterior 

 portion of the body, however, is often spoken of as being least modified. 



The remaining space between the various organs and the peripheral 

 granular layer is occupied by vesicular parenchyma (Plate 2, Fig. 25, 

 pa^ench.), which usually exhibits a quite regular network, the meshes of 

 which undoubtedly correspond to the cells from which the tissue has 

 been derived ; the walls forming the network are obviously the walls of 

 those cells. I have found no nuclei in the vesicular parenchyma : the 

 bodies which in transverse sections appear at first sight to be such, prove 

 on closer examination to be the cut ends of parenchyme muscle fibres 

 (Fig. 25, mu. rtr.). In the posterior portion of the body there are 

 scarcely any traces of granular cytoplasm, the meshes in life being filled 

 probably with a watery fluid. In the anterior portion of the body con- 

 siderable granular substance may be found in the meshes of the paren- 

 chyma, and the vesicular parenchyma often passes more or less gradually 

 into the peripheral granular layer. The meshes are of various sizes, some 

 of them being as large as the cells ensheathing the crura, but the most 

 of them smaller and, as a rule, more or less compressed. 



Anterior to the vitellaria the meshwork is not so regular as in the 

 posterior portion of the body, and in front of the ventral sucker occasional 

 cells similar to the subcuticular cells occur scattered throughout the ve- 

 sicular parenchyma. The longitudinal parenchyme muscle fibres which 

 form the retractors of the appendix are a characteristic feature in the pos- 

 terior portion of the body. They pass through the walls of the meshes 

 of the vesicular parenchyma. 



Many of the dorso-ventral parenchyme muscle fibres are provided with 

 a centrally situated nucleus surrounded with granular cytoplasm, while 



