4 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



two series of fibres, which cross each other at nearly right angles. The 

 distance between the fibres is such that the meshes are from two to four 

 times as wide as the fibres. Fibres of all three layers are occasionally 

 seen to branch. 



In the appendix the body muscles are less strongly developed than 

 in the trunk, and the oblique muscle fibres are lacking. 1 find, as 

 stated by Juel ('89), that the relative positions of the circular and 

 lonofitudinal muscle fibres are the reverse of those found in the trunk, 

 the longitudinal fibres of the appendix being next to the cuticula (Plate 2, 

 Fig. 14). Juel calls attention to the agreement in position between the 

 muscles of the appendix and those of the excretory vessel, and suggests 

 that the muscles of the two have developed in connection with each other. 

 I think his suggestion quite pertinent and believe that the inversion of 

 the muscle layers of the appendix is significant as to the method in which 

 the appendix originated. Pratt ('98) believes that the appendix is the 

 whole, or a part, of the excretory vesicle in an evaginated state. la 

 young specimens of Apoblema [Ilemiurus] appendiculatum which are as 

 yet without an appendix he finds at the posterior end of the body what 

 he calls an appendicular vesicle, a sack lined with a high columnar 

 epithelium. At its posterior end, this sack opens to the exterior by a 

 pore guarded by a sphincter, while into its anterior end the excretory 

 vesicle opens. Later this appendicular vesicle becomes evaginated to 

 form the appendix and its epithelium is sooner or later shed. 



In Hemiurus crenatus and other appendiculate distomes, so far as they 

 have been described in this particular, the excretory vesicle is provided 

 with longitudinal muscle fibres which lie next to its structureless lining, 

 and surrounding these are circular muscle fibres. Upon evagination of 

 the excretory vesicle the muscles of its wall would appear as body 

 muscles, the outer layer consisting of longitudinal fibres and the inner 

 layer of circular fibres, which is precisely the condition found in the 

 appendix, but the reverse of that found in the rest of the body. It 

 seems to me, therefore, that Pratt's view as to the origin of the appendix 

 furnishes a satisfactory explanation of the difference between the ar- 

 rangement of the muscle layers of the appendix and those of the trunk, 

 — a condition for which it seems difficult to offer any other explanation. 



The muscle fibres of the suckers are well developed, and both the 

 radial and peripheral fibres show a clear central area, while the periphe- 

 ral portion of the fibre shows a fibrillar structure. Side views of fibres 

 show clearly a longitudinal striation, and in cross section the peripheral 

 portion of each fibre has the appearance of minute dots. 



