1!,|1 I Watson: The Genus Gyrocotyle. 411 



sets of nerves to the muscle layers, which can always be distin- 

 guished from one another. 



The central nervous system lies within (mediae! to) and 

 among the inner longitudinal muscle-fibres, and within (mediad 

 to) the large longitudinal excretory canals. Tt consists of the 

 following parts : 



1. Two lateral longitudinal stems. 



2. An anterior bridge commissure and an anterior ring com- 

 missure, in the acetabular region. 



•'!. A posterior bridge and two ring commissures, joined by 

 eight longitudinal connectives. 



The nervous system was first recognized by Wagener (1852), 

 who saw only the anterior commissure. Monticelli (1889a) and 

 Spencer (1889) recognized the posterior commissure. According 

 to 5-jpencer, this commissure is continuous around the canal, form- 

 ing a complete ring surrounding the canal ; the dorsal half of the 

 commissure extending farther posterior than the ventral half. 

 According to Monticelli this is not the case. He found that on 

 the ventral surface the two parts did not unite, but merely ran 

 alongside each other, then separating passed posteriorly, each 

 ending independently in the margin of the "Trichter. " Both 

 these investigators agree in placing the anterior, heavy and in- 

 dubitable part of the commissure on the same surface as the 

 canal opening, i.e., the ventral face (dorsal of Spencer and .Monti- 

 celli). This puts the anterior (acetabular) commissure on the 

 opposite surface from the posterior (scolex) commissure. 



Lonnberg (1891) described both commissures as lying on the 

 ventral surface. He found the posterior commissure to be a 

 bridge, not a complete ring, posterior to which the longitudinal 

 stems are continuous to the margin of the funnel, where each 

 breaks up into many branches, which probably form by their 

 anastomoses a ring about the margin of the funnel opening. 

 Thus his account differs from Monticelli 's in (1) the presence 

 of a marginal ring at the posterior extremity, and (2) the absence 

 of any r statement of the near approximation of the posterior 

 nerve stems to each other posterior to the commissure. It differs 

 from Spencer in the first point, and also in the absence of any 

 indication of the completion of the posterior commissure to form 



