Life-history anfl Anatomy of the Appendiculate Distomes. 363 



an environment a worm, whose integument consists of the hypodermis 

 and cuticula of most Invertebrates, could not exist. And so it has 

 happened that in the process of the development of the race of Tre- 

 matodes the usual integument of Invertebrates has been abandoned, 

 and the peripheral portion of the parenchyma has consolidated to form 

 a thick, cuticula-like, but often fibrous or granular membrane on the 

 external surface of the animal. It does not seem probable to me, 

 however, that even its great thickness would alone be sufficient to 

 protect the integumentary membrane of these worms from the dangeis 

 of their environment. The integumentary glands, which are universal 

 among Trematodes, have, in my opinion, as their principal function 

 the secretion of a substance which shall neutralize the action of the 

 corroding fluids in which the animals are often habitually submerged. 



The cuticula of Trematodes must have the power of rapid growth. 

 The worm increases in size many fold from the time it arrises in its 

 final host until it attains full size, during which time its cuticula in- 

 creases in thickness proportionately. I think, too, that notwithstanding 

 the probable protective effect of the secretions of the integumentary 

 glands, there must usually be constant wear and destruction of tissue 

 on the outer surface of the cuticula. The granular or vesicular ap- 

 pearance (see within, p. 356) which often marks this surface in the 

 worm under discussion, and which has been noticed in other Trema- 

 todes, I take to be a symptom of progressive disintegration. If this 

 he a correct diagnosis then constant and rapid growth and regeneration 

 of tissue must take place on the inner surface of the cuticula, in 

 order to make good the losses being constantly sustained. 



When we ask what is the nature of this process of growth, and 

 the source of the new matter which is added, we get very conflicting 

 answers from diff"erent observers. Brandes (1892) sees in the sub- 

 nuiscular cells of the parenchyma single -celled integumentary glands: 

 they are connected with the inner surface of the cuticula by long ducts 

 which pass through the superficial muscle-layers, and secrete the new 

 matter by the addition of which it grows. An insuperable difficulty 

 of this theory, it seems to me, is the fact that in appendiculate Dis- 

 tomes, as is shown in the present discussion (see pag. 370), submuscular 

 cells are present in the trunk of the worm but not in the appendix 

 (PL 25, Fig. 3 s. c), and yet the appendix possesses a cuticula of the 

 same structure and thickness as the trunk. Looss (1894) also objects 

 to it for the reason that a thick and rapidly growing structure like 

 the cuticula could hardly be produced by the relatively small number 



