106 PROCEEDINGS OF THE ACADEMY OF [PaRT I 



I was then enabled to study this very interesting organism at some 

 length, and should like to discuss here some of the most important 

 points of its structure. 



Regarding the identity of my specimens: Lemmermann speaks 

 of "cells longer than broad, in optical longitudinal section rounded 

 with 5-6 angles," while in the Ariana specimens the breadth of 

 the body was normally at least equal, and very often superior, to 

 the length; and at the same time the anterior part was broader 

 than the posterior (Plate V, fig. 1); in optical longitudinal section 

 the form, rather than "rounded with 5-6 angles," was that of a 

 top with a truncated point. In short, this Ariana form was but 

 in a very general manner in accordance with Lemmermann's de- 

 scription; but as this very small organism is difficult to study, and 

 is perhaps subject to great variation in its shape, it seems best 

 at present to consider the two forms as representing the same or- 

 ganism. 



Pteridomonas in form, as we just said, is more or less that of a 

 top; the size is about S-O^jl, often rather less, and rarely more. In 

 the anterior part of the body we find, right and left, two small con- 

 tractile vesicles, and also, in the posterior part, two other vaculoes, 

 but of a special nature; the body, in fact, seems to be hollowed out 

 to form two small depressions, one at the left and the other at the 

 right, with a clear space or vacuole in the center; they hardly look 

 like contractile vesicles, especially since these depressions are some- 

 times wanting, yet sometimes I have seen the vacuoles suddenly 

 collapsing, as contractile vesicles generally do. The flagellum, whose 

 length is equal to three or four times that of the body, is relatively 

 strong, and easily seen. From the middle of the anterior part of 

 the body, which is itself somewhat flattened or even excavated, 

 this flagellum points directly upwards during the first part of its course, 

 then curves right or left, and when vibrating becomes hardly visible. 

 At its point of fixation it is provided with a basal granule, very 

 small and mostly quite invisible, Ijut which I could detect once on 

 a gradually and strongly compressed specimen. 



In Lemmermann's diagnosis we noted the following lines: "One 

 flagellum, and rarely besides, two slowly vibrating accessory fla- 

 gella. At the base of the flagellum, a crown of fine radiating pseu- 

 dopodia." We must treat at some length of these rather important 

 organs. 



The central flagellum is normally surrounded by a crown of very 

 fine threads, perhaps about one dozen in number, quite rigid and 

 motionless, which point straight into the surrounding liquid, at an 

 angle of about 45° from the central flagellum. These threads, 

 which have generally been described as pseudopodia, and indeed 



