1911] Watson: The Genus Gyrocotyle. 385 



body. The outer surface of the membrane is bounded by a fine, 

 deeply stained layer of ragged fibres (ext. cut. I.). This is seen 

 only in perfectly preserved specimens. Vesicles occur frequently 

 in the membrane. There are also breaks in the margin, resem- 

 bling sections of the "pore-canals" figured by Blochniann I 1896 ). 

 The thickness of this membrane varies in different regions, being 

 slightly less on I he lateral folds and on the surface of the aceta- 

 bulum. The average thickness on the surface of the body proper 

 is about 3.8^; on the lateral folds, etc., about 2/x. This membrane 

 will be referred to as a "cuticula," following the common usage 

 among writers on the subject; but the word is not used in its 

 strict sense, as denoting a body-covering derived from an under- 

 lying epidermis. The cuticula of Gyrocotyle may or may not be 

 derived from the "subcuticular" layer referred to below; struc- 

 tural relations indicate that it is, but in the opinion of the writer 

 the question can only be settled by embryological data which arc 

 not available. 



Lonnberg (1891) describes a two-layered cuticula, agreeing 

 in general with the appearance of that of G. finibriata; but he 

 describes the fibres seen in the "Ilauptschichte" or principal 

 layer, as running perpendicular to the surface. He finds this 

 only in sections in which the cuticula is torn or affected by 

 reagents; from appearances presented in similar sections, the 

 writer thinks he was probably dealing, not with the cuticular 

 layer, but with a layer of perpendicular fibres lying beneath the 

 cuticular muscles, to be described later. 



Immediately beneath the cuticula (cut. trans., pi. 42, fig. 

 55) is a layer of cuticular muscle-fibres running round the body 

 at right angles to its long axis (cut. trans., pi. -42, fig. 58). Close 

 beneath this layer lies a set of longitudinally arranged muscle- 

 fibres (cut. long.) ; these are heavier than the transverse fibres. 

 These layers of muscles are distinguished as the cuticular muscu- 

 lature. Some distance below these muscle fibres is an irregular 

 layer of large nuclei, about twice the size of the ordinary paren- 

 chyma nucleus, resting on and often imbedded in the outer 

 transverse layer of body musculature (sub. cut.). This layer 

 may be one or two nuclei deep, or it may be from five to eight 

 deep. In the neighborhood of a spine the layer becomes three 



