10 s. GOTO. 



almost iiiiifoniily disfribnted on the dorsal surface, while in Trisf. 

 siNiKihnii they are mostly confined to near the lateral margins of the 

 venti-al side ; and in both species they measure on the average 0.011 

 înm. in lieight. They are probably tactile organs. 



In sections the papillae of the first kind (PI. XXIV, fig. 2) are 

 seen to be simple elevations of the investing membrane together with 

 the underlying niesenchyma which presents, however, a somewhat 

 different appearance from that of the more intei-nal parts, and will be 

 described under the mesenchyma. These papillae are always traversed 

 lengthwise by the terminal ramifications of the dorso-ventral muscular 

 fibres, one of which usually ends at tlie very apex. The papillae of 

 the second kind are distinguished not only by their minute size but 

 also by the total absence of muscular fibrils ; and although I have not 

 been able to demonstrate in them any ner\'ous fibril I belie\'e it will 

 be found out by the application of appro|)riate methods. 



The total thickness of the investirjg membrane is in Microcotyle 

 usually a little less than 0.005 mm. ; in Axinc lietfroccrea a little less 

 than 0.004 mm. ; in Diclidojilioiri 0.005 mm. ; in Hexacotijh acuta 

 and (k-tocotijJe minor 0.004 mm. ; in MonocotijJe 0.008 mm. ; and in 

 Oiichocotiih' O.OOP) mm. — 0.004 mm. It sliould however 1)e borne in 

 mind that tlie thickness of the investing membrane varies considerably 

 in different parts of the body. Thus, in Tr. ovale it is O.OOo mm. on 

 the ventral side and 0.01 1^ mm. in the anterior part of the dorsal 

 side ; again in Tr. simialum the thickness varies from O.OOo mm. on 

 the dorsal side of the posterior sucker to 0.01 mm. on the dorsal side 

 near the anterior sucker. It is also to Ije noted that the thickness is 

 sometimes less on the dorsal tlian on the ventral side, ej/. in 

 (hicliocoti/h'. 



Having described the investing membrane as it is according to 

 my own observations, I may now refer to some views relating to its 



