198 HENRY LESLIE OSBORN 



oblique. I have recently made a re-examination of the sections 

 on which my paper of 1904 was based to test the possibility of 

 the coat being present in Cotylaspis; as a result I am entirely con- 

 vinced that there is no outer longitudinal muscular layer. It thus 

 seems safe to conclude that Clinostomum is pecuHar in the pos- 

 session of this layer, though a similar may perhaps be found later 

 in some other forms. My observations of the other coats also 

 confirm those already reported by Looss. The fibers of the cir- 

 cular coat lie in several layers (fig. 7) ; they are very small, though 

 larger than those of the outer layer, measuring 0.0012 mm. They 

 do not fall into groups or bundles like those of the inner longitu- 

 dinal layer. These fibers are seen in sections generally at various 

 levels between the sub-cuticular excretory cavities, which thus 

 seem to occupy an area produced by the expansion of that part 

 of the body wall, in correlation with the presence of these 

 cavities. 



The inner longitudinal muscles lie much deeper than usual. 

 Instead of lying quite near the cuticle as they do in many cases, 

 and in close contact with the circular muscles, they are located 

 here, as show^n in fig. 6, below the vessels of the excretory system 

 at a distance of 0.04 mm. from the outer muscles. The inner 

 muscles are thus seen to be pushed down to a considerable depth 

 below the bottom of the cuticle near which they usually lie. This 

 departure from the ordinary arrangement is clearly a structural 

 adaptation correlated with the presence of the sub-cuticular 

 vessels. We may further perhaps regard the presence of the outer 

 longitudinal nmscles as a part of the same adaptation; they may 

 have been developed thus near the surface to offset the disad- 

 vantage due to the increased distance of the inner longitudinal 

 layer from the cuticle. 



The inner longitudinal muscle fibers are very distinctly grouped 

 into bundles alternating with intermediate areas from which they 

 are absent. Fig. 7, mi, shows one of these bundles in cross sec- 

 tion; it is made up of a cluster of fibers without other muscles in 

 close proximity. The fibers of the inner longitudinal muscle 

 differ in size, as can be seen in fig. 7; the largest ones are much 

 larger than those of the circular muscle, measuring 0.004 in diam- 



