Life-hîstory and Anatomy of the Âppendiculate Distomes. 369 



and 493) has found that young and developing muscles in Trematodes 

 are spindle-shaped with nuclei in their centers and with attenuated 

 ends; he has also showed that as the muscle grows, its walls thicken, 

 its nuclei disappear, it becomes slender and compact, and the center 

 or core may finally appear transparent under ordinary stains. We 

 have seen in the younger worms we have investigated that the retractor 

 muscles are massive and little attenuated while in the older worms 

 they are rapidly becoming slenderer. A continuance of this process 

 would result in the development of the form and condition found by 

 JuEL in the adult animal. 



The wall of the appendix contains superficial muscle-fibres. They 

 are so delicate, however, in the young worm, and the worm itself is 

 so minute, that their arrangement could not be satisfactorily deter- 

 mined. In the adult worm Juel finds two distinct layers of muscles, 

 an outer longitudinal and an inner circular layer, the reverse order, 

 thus, of their arrangement in the trunk. 



The muscles of the sucker and the pharynx appear with a very 

 simple arrangement. I could detect three layers or systems of them : 

 the thickly set radial fibres which constitute the main mass of the 

 organ; the outer superficial layer of delicate, parallel fibres which 

 skirt the outer wall of the organ between the bases of the radial 

 fibres and which appear in a section as minute dots ; the correspond- 

 ing inner superficial layer of fibres which skirt the inner wall of the 

 organ (PI. 26, Fig. 10 and P. 27, Fig. 13). 



Parenchyma. 



The parenchyma of this worm presents the conditions usual in 

 digenetic Trematodes; it is a highly vacuolated tissue with nuclei 

 lodged near the cell-walls. The cells are very irregular in outline 

 and are usually not elongated in any one axis ; those, however, which 

 surround the retractor appendicular muscles show a decided tend- 

 ency to become elongated in the direction of the strain of these 

 muscles (PI. 26, Fig. 6). The diameter of the largest vacuoles is 

 0.006 mm ; that of the cell-nuclei is 0.0015 mm. These latter are 

 small bodies, relatively numerous, which can easily be distinguished 

 from the nuclei of the retractor muscles and of the submuscular cells. 

 The subcuticular layer of the parenchyma (intermusculäre Aussen- 

 schicht of Braun or ecto-parenchyma of Brandes), which is often 

 present in muscular Trematodes, is absent in the trunk of this worm ; 



