GENUS PTYCHOSTOMUM. 541 



Pleurochilidium strigilatum, Stein. 



Body subreniform, depressed, the anterior extremity narrowest, obliquely 

 truncate, turned toward the right, the margin of this region crenulate ; 

 endoplast spherical, subcentral ; contractile vesicle situated posteriorly, 

 close to the right-hand border ; trichocysts large and scattered. Length 

 1-700". HAB. Fresh water, among Lemna. 



GENUS XI. PTYCHOSTOMUM, Stein. 



Animalcules free-swimming, persistent in shape, not encuirassed, 

 asymmetrical, more or less ovate, depressed or plano-convex, the right- 

 hand margin convex, the left one concave, the posterior margin obliquely 

 truncate, oral aperture ventral, located towards the anterior extremity, anal 

 aperture situated on the left-hand side of the posterior extremity. Occur- 

 ring as endoparasites within the intestinal viscera of various Annulosa 

 and Mollusca. 



Ptychostomum ssenuridis, Stein. 



Body subovate, slightly depressed ; the right border convex, the left one 

 indented centrally; the ventral surface flat, the dorsal one convex, the 

 obliquely truncate posterior extremity curved towards the left in a spur-like 

 manner ; oral aperture subtriangular or acetabuliform, antero- ventral ; anal 

 aperture tubular, located near the left posterior border, enclosing a bristle or 

 lappet-like structure ; cuticular surface longitudinally striate, entirely ciliate, 

 cilia long and fine ; endoplast elongate-ovate, transversely placed ; contrac- 

 tile vesicle situated close to the right posterior margin, rosette-shaped at 

 systole. Length 1-288". HAB. Alimentary tract of Scsmiris variegata. 



The bristle or lappet-like organ described by Stein as situated within the anal 

 tube, is so anomalous that it appears an open question whether the so-called acetabu- 

 liform mouth is not an independent sucker or acetabulum, as in Haptophrya, 

 permitting the animalcule to fix itself to any desired point of the intestinal wall of 

 its host, and the so-called anal tube, with its bristle or undulating membrane, 

 an oral system corresponding closely with that of Panophrys, Microthorax, or 

 Trochilia. 



Ptychostomum paludinarum, Stein. 



Similar in shape to P. sanuridis, but the posterior extremity on the 

 right side obliquely truncate, bounded by a narrow undulating border, 

 which is at its left angle reflected inwards and projects to a short extent 

 upon the ventral surface ; oral aperture anteriorly located, cleft-like, occupy- 

 ing a transverse semicircular furrow that is parallel with the anterior margin 

 of the body ; endoplast elongate-oval, subcentral ; contractile vesicle single, 

 spherical, located immediately behind the endoplast. Length 1-168". 



HAB. Intestinal tract of Paludina impara and P. similis. 



Diesing, ' Prothelminthen,' 1866, has proposed to distinguish this species 

 generically from the preceding animalcule under the title of Hysterocineta, but until 



