Life-history and Anatomy of the Appendiculate Distomes. 367 



in cross-section shows a colorless core surrounded by a deeply staining 

 peripheral portion; the average diameter of each fibre is 0.0026 mm. 

 A layer of oblique fibres is present beneath the longitudinal muscles; 

 as in the case of the circular muscles, the separate fibres cannot be 

 distinguished from one another ; the diameter of the layer is 0.002 mm. 

 Oblique muscle-fibres do rot appear in the forward and hinder portions 

 of the body. Parenchymatous muscle-fibres are very poorly developed 

 in the forward and middle portions of the worm; they are represented 

 only by a few strands which pass from the inner surface of the aceta- 

 bulum to the dorsal l)ody-wall, and a few weaker strands which act 

 as retractor muscles of the oral sucker. In the hinder part of the 

 body, however, parenchymatous muscle-fibres are very prominent and 

 important muscles and form the retractors of the appendix. Jukl 

 (1890) is of the opinion that these appendicular retractors are modi- 

 fied longitudinal muscle - fibres , and his judgment in the matter is 

 entitled to greater weight than mine as he examined adult worms, 

 where the conditions would probably be plainer, and, also, worms of 

 several different species. I think it will be acknowledged, however, 

 that in the young worm here under discussion, where primitive con- 

 ditions would naturally be found, these muscles have every appearance 

 of belonging to the parenchymatous muscle-system (compare Fig. 5, 6 

 and 7, PI. 26). This is supported also by the fact that longitudinal 

 fibres are present in the hinder part of the body, although very much 

 smaller, yet in the same order and arrangement as in the other 

 portions of it. 



These retractor muscles are strikingly large, elongated , club- (»r 

 si»indle-shaped fibres often with anastomosing ends (PI. 26, Figs. r>, 

 6 and 7). They are richly nucleated, the smaller fibres containing 

 one nucleus, the larger ones, two, three or more; these nuclei arc 

 invariably in the center of the cells. The insertions of these muscles 

 are in the cuticula of the appendix, their origins are either in the 

 cuticula of the trunk or in the parenchyma near the submuscular 

 cells. Those fibres which are located near the extremity of the trunk 

 and pass thence to the base of the appendix usually arise in the cuticula 

 of the trunk. The great majority of fibres, however, including all those 

 whose insertions are farther towards the apex of the appendix, arise, not 

 in the cuticula, but in the parenchyma near it; these fibres are usually 

 club shaped, the thick, blunt ends being near the submuscular cells, the 

 attenuated ends terminating in the appendicular cuticula (PI. 26, Figs. 5 



