10 S. GOTO. 



almost uniformly distributed on the dorsal surface, while in Trist. 

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

 ventral side ; and in both species they measure on the average 0.011 

 mm. in height. They are probably tactUe 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 mesenchyma which presents, however, a somewhat 

 different appearance from that of the more internal 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 the 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 nervous fibril I believe it will 

 be found out by the application of appropriate methods. 



The total thickness of the investing membrane is in Microcotyle 

 usually a little less than 0.005 mm. ; in Axine lieterocerca a little less 

 than 0.004 mm. ; in Diclidophora 0.005 mm. ; in Hexacotyle acuta 

 and Octocotijle minor 0.004 mm. ; in Monoeotijle 0.008 mm. ; and in 

 Onchocotijh 0.003 mm. — 0.004 mm. It should however be borne in 

 mind that the thickness of the investing membrane varies considerably 

 in different parts of the body. Thus, in Tr. ovale it is 0.003 mm. on 

 the ventral side and 0.012 mm. in the anterior part of the dorsal 

 side ; again in Tr. sinuatum the thickness varies from 0.003 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 be noted that the thickness is 

 sometimes less on the dorsal than on the ventral side, e.g. in 

 Onchoeotjjle. 



Having described the investing membrane as it is according to 

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



