368 H. s. PRATT, 



6 and 7). I expected, at first, to find fine fibres running from these 

 blunt ends through the submuscular and muscular layers to the cu- 

 ticula, and that the real origin of the retractor muscles would be 

 here, but I have looked for any such fibres in vain. The retractor 

 muscles arise apparently in the parenchyma. 



The actual shape of these fibres depends, however, entirely on 

 the position of the appendix, whether it is extended or not, and, also, 

 in some degree, on the age of the worm. In the youngest worms, 

 those which were taken in the Copepod and in which the retractor 

 muscles can never have been functional, as the appendix has not yet 

 begun to extend itself, they are thick structures throughout a greater 

 part of their extent (PI. 26, Fig. 5). In the older worms, which have 

 begun to extend the appendix, the retractor muscles are much slighter. 

 In Fig. 6, although the animal represented is still in the Copepod, 

 it is one of the larger worms and will probably soon leave its host; 

 it has just cast its epithelium and the retractor muscles have plainly 

 aided in the operation. In Fig. 7 these muscles are also more at- 

 tenuated and lack the massive appearance of those in Fig. 5. 



Where the appendix is extended the retractor muscles are much 

 changed. They are naturally a great deal stretched and have become 

 slender, attenuated structures throughout their entire extent. Their 

 inner ends remain knob-like, however, and blunt, and it will be seen 

 (PI. 25, Fig. 3), have not been stationary but were carried along with 

 the appendix when it evaginated, and are now contained quite within 

 it, extending but little into the trunk of the worm. 



The retractor muscles all appear as distinct and separate, although 

 sometimes anastomosing, fibres, and are never grouped in bundles. 

 The average diameter of the larger of them is 0.007 mm ; that of the 

 nucleus is 0.0025 mm ; the length of the longest fibres in the extended 

 appendix is 0.05 mm. 



A comparison of the results of my study of these retractor muscles 

 in the young worm with those of Juel (1890, p. 19) of the same 

 muscles in the adult is interesting. According to his observations 

 the retractor muscles in the adult ApoUema have a structure similar 

 to that of the longitudinal fibres, described above: each fibre is a 

 narrow, non-nucleated strand, the cross-section of which shows a 

 colorless center surrounded by a deeply staining periphery. 



These muscles, then, in the youthful animals under discussion 

 must still be in the process of development. Poirier (1885, p. 492 



