LEIDY'S DESCRIPTIONS OF DISTOMA. 307 



The posterior contractile ventricle or sac is an oblong cavity with muscular 

 parietes, the longitudinal strise of which are quite distinct. It communicates with 

 the exterior by a posterior infundibular orifice, and at its anterior part by means of 

 two short canals with the dilated or bulbous commencement of the vascular canal on 

 each side. When fully contracted it forms a narrow longitudinal canal. Whether by 

 expansion this sac takes within it some of the liquor pericardii in which, it lives, and 

 upon contraction sends it throughout the vascular canals, I did not determine. The 

 vascular canal commences posteriorly a little in advance and laterally to the 

 contractile ventricle, and proceeds forwards in a very tortuous manner as far as the 

 side of the oral acetabulum, when it turns back and almost retraces its steps in the 

 same tortuous manner to the posterior extremity of the body, where it terminates in 

 fine branches. In its course backwards it gives off numerous lateral branches which 

 divide and subdivide among the organs and tissues of the body, and anastomose with 

 the corresponding branches of the other side. 



The oesophagus after leaving its bulb is short ; the intestinal rami first of all turn 

 obliquely forwards and outwards and then pursue a tortuous course backwards. 

 They are irregularly cylindrical and contractile. The contents consist of a fine 

 granular matter, without the spiculse of the first stage. 



The aperture situated between the ventral acetabulum and posterior extremity, is 

 often closed by contractile power as not to be distinctly visible. It opens into what 

 appears to be the visceral cavity, the outline of which is distinguishable just without 

 the course of the intestinal rami. 



In the change from the second to the third stage the animal loses its sluggish 

 character, contracting and lengthening its body extremely, and protruding the 

 acetabula,the ventral one of which appears pediculated from the cylindrical prolongation 

 of that part of the body to which it is immediately attached. The posterior contractile 

 sac becomes inactive, is cut off from its connection with the vascular canals, and is 

 often indistinct. How the animal escapes from the pericardial sac I am unable to say ; 

 I have examined over sixty Helices without ever being able to discover anything 

 like a perforation in it. 



In its third stage I have never seen the Distoma in the pericardial sac, but always 

 in the tissues attached to the foot or in the substance of the latter itself. In this 

 stage it is always more elongated than in the others. 



Instead of their being two, there are now four principal vascular canals on each 

 side of the body, and these having a more or less tortuous course forwards and 

 backwards form a very complex plexus. From them pass off numerous smaller 

 branches, varying much in their course, and distributed by subdivision to all the 

 body. In the longest of the principal vessels (from the dot a to 5 in fig. 8,) the 



interior surface is furnished with actively moving vibrillss. This movement of the 



7s 



