STUDIES ON THE ECTOPAEASITIC TEEMATODES OF JAPAN. 3 



A. Anatomy and Histology. 



1. External Fwm of the Body. 



Broadly speaking, the form of the body is that of the blade of a 

 leaf with a rounded apex ; and as the leaf varies from an orbicular 

 to a linear or lanceolate shape, so the body of ectoparasitic Trematodes 

 varies in form between the same extremes. In Microcotyle the body 

 is generally elongated and lanceolate or fusiform, the posterior end 

 being rather pointed. In some species of this genus the halves 

 of the body are asymmetrical, one being longer than the 

 other, so that the axis of the body forms a curve or even a 

 crooked line ; e.g. in M. reticulata (Pi. I, fig. 5) and M. sciaenae 

 (PI. II, fig. 6). 



In cross-sections the body of Microcotyle presents the form of an 

 ellipse, of which the minor axis becomes greater and the major axis 

 much shorter as the section approaches the anterior end. In the pos- 

 terior portion of the body where the suckers are present, the cross-section 

 is often semicircular in outline, the diameter being the ventral side. 



In Axine (PI. VII), one side of the body is always longer than 

 the other, and the posterior portion of the longer side makes an angle 

 with the anterior part, so that this portion looks like the posterior 

 naargin of the body, and has actually been so regarded by preced- 

 ing writers. But that it is really a part of the lateral margin of the 

 body seems to me beyond doubt both from the presence of suckers 

 on the other side and from the course of the principal nerves and the 

 excretoiy vessels to be described further on. In ^. aberrans the 

 sucker-bearing portion of the longer side is perfectly straight and 

 makes an acute angle with the anterior part, so that one is tempted to. 

 regard it as the posterior border of the body ; but here too there is a 

 sucker on the other side. In accordance with the general asymmetry 



