366 H. s. PRATT, 



Much simpler than Looss' cambium-theory, and yet to me per- 

 fectly sufficient, is the explanation suggested by Leuckart and above 

 referred to. He points to the fact that connective - tissue structures 

 in the lower animals may often readily assume a cuticular character. 

 Indeed, throughout the animal kingdom such structures commonly form 

 protecting membranes about ditïerent organs. It is, consequently, no 

 strange thing that the parenchyma of Trematodes and Cestodes has, 

 in the course of the evolution of these worms, acquired the property 

 of forming a thick cuticula-like membrane on its outer surface to 

 protect them from the peculiar dangers of an environment in which 

 existence, with no external covering save the usual Invertebrate - in- 

 tegument, would be impossible. 



I have still to mention one other theory of the origin and mor- 

 phological significance of the cuticula which has had great vogue lately: 

 I refer to the theory of Ziegler (1883), Bieiiringer (1884), Braun 

 (1892, p. 590) and Monticelli (1892, 1893) that the cuticula is a 

 metamorphosed epithelium, the cellular structure of which has dis- 

 appeared. These authors base their theory largely on the occurrauce 

 of minute structures in the cuticula which they interpret to be the 

 remains of disintegrating nuclei. I have already (p. 357) referred to 

 these nucleus-like structures and repeated Looss' explanation of them : 

 his proof of their pathological character amounts to a demonstration, 

 and I need only refer to his works in this connection (see Looss, 1894, 

 p. 116—119, 134). 



fliiisculaturc. 



The musculature of the youthful Apoblcma appcndiculatii»! , as 

 observed by me, shows certain interesting variations from its usual 

 character in Trematodes, and also from that in adult specimens of 

 the genus as described by Juel (1890). In the forward and hinder 

 portions of the body the superficial muscle-layers are of extreme 

 delicacy and often difficult to demonstrate. In the midde portions of 

 the body, however, these muscles present the usual appearance. Im- 

 mediately beneath the cuticula and not separated from it by an ecto- 

 pareuchymatous layer are the circular muscle fibres (PI. 26, Fig. 9); 

 they are non-nucleated and have a thickness of 0.0013 mm, and, as 

 is often the case in small Trematodes, the separate fibres cannot be 

 distinguished from one another. Beneath them are the longitudinal 

 muscles ; they constitute a single row of fibres which are separated 

 from one another by spaces; they are non-nucleated, and each fibre 



