452 W s. NICKERSON, 



I proceed now to treat somewhat m detail of the different systems 

 of organs found in the young Stichocotyle. The specimens which I 

 collected in June showed in several details of structure a little more 

 advanced state of development than those obtained in April. As 

 Cunningham's specimens were all obtained «luring the winter or early 

 spring, it is only natural that I should be able to make out a number 

 of features of greater or less morphological significance which he did 

 not observe. 



Cuticula. 



The outer covering of the body is a thick, nearly homogeneous, 

 elastic coat, which resembles closely that of other Trematodes. Its 

 thickness, however, is rather remarkable averaging 8 — 11 // over the 

 greater part of the body. Within the cavities of the suckers, however, 

 it is much thinner, being not over one half the normal thickness. At 

 the posterior end of the body it is also somewhat below the average 

 thickness. It is entirely destitute of spines or bristles or hooks. Pore 

 canals also are not present. When the living worm is considerably 

 elongated this coat has a smooth regular outline, but in the normal 

 serai-contracted state it is seen to be thrown into distinct, more or 

 less regular, transverse wrinkles which appear in longitudinal sections 

 as crenations and give a ringed appearance to the body. In longi- 

 tudinal sections these crenations are seen to be due entirely to undul- 

 ations of the external surface, the inner surface remaining unatlected 

 and being everywhere in close contact with the underlying musculature. 

 Surface views and tangential sections (PI. 30, Fig. 18) show that 

 these transverse wrinkles are not due to simple circular constrictions 

 at equal distances apart and extending in each case entirely around 

 the worm, but that instead there are considerable inequalities in them ; 

 the outlines of the individual furrows are far from even or regular, 

 and few if any of them conii)letely encircle the body. These facts 

 show that the material of which the outer layer is composed is flexible, 

 being stretched to accommodate the elongations of the worm and so 

 com[)ressed as to throw its surface into wrinkles when the animal is 

 in a contracted state. 



Very divergent opinions are still held by naturalists concerning 

 the nature of this layer and its source is distinctly ])roblematical. 

 Several theories have been advanced to account for it. 



First, the earlier theory, that this external layer is a basement 

 membrane derived from ;ui epidermis which has been lost, is now 



