154 ENTOZOA. 



symmetry is not extended beyond the general disposition of the 

 main branches. In no case have I witnessed any intestinal anas- 

 tomoses. Several observers have represented the secondary and 

 tertiary branches as becoming smaller and smaller as they proceed 

 towards the margin, also conveying the idea, that the ultimate 

 branchlets are extremely fine. This is decidedly an error, and 

 shows that their descriptions have not been taken jfrom artificially 

 injected specimens. The truth is, that the ultimate branches, as 

 well as their ccecal ends, are only a very httle smaller than the 

 primary intestinal trunks. Histiologically speaking, the alimentary 

 tubes consist of two layers ; an outer fibrous wall made up of thin 

 filaments, and an inner, cellular layer, consisting of columnar ceUules, 

 which, when viewed in fii-ont, present a polyhedral figure. According 

 to Leuckart, they have a breadth of i^ of an inch. The fluid intes- 

 tinal contents are, for the most part, made up of bile derived from 

 the gall ducts of the host, in which there may also be frequently 

 observed numerous epithelial scales as well as blood corpuscles 

 derived from the same source. These matters are therefore, mani- 

 festly, the principal source of nutriment by which the fluke is 

 supported ; but it is also probable that other nutrient fluids, 

 absorbed by the skin, contribute their quota towards the growth 

 and nourishment of the animal. 



In strong contrast to the digestive system of canals, there is 

 another series of tubes lying in closer proximity to the dorsal 

 surface of the body, and presenting features altogether distinctive. 

 I here speak of the so-called aquiferous or water-vascular system. 

 In favourable examples of the worm, every p^t of this set of 

 vessels may be viewed by the aid of a pocket lens, even in cases 

 where there has been no attempt made to inject them. In such 

 examples I find one central, longitudinal trunk (^" broad), extend- 

 ing backwards from the upper third of the body in a direct line to 

 the extremity of the tail, at which point it terminates in an open 

 foramen caudate (Plate XI., Fig. 3). At the spot coinciding with 

 an imaginary line marking the upper third of the body, the single 



