as Infusoria flagellata. 251 
For the sake of accumulating and multiplying diagnostic 
characters that shall serve us hereafter as discriminative points 
in determining the classificatory relations of Flagellata, it is 
most desirable that every critical study of one of these forms 
should be carefully recorded, even to the minutest details. 
On this account, therefore, and particularly in the present _ 
connexion, notwithstanding that this species is so frequently 
met with, and apparently so well known, it will not be out of 
place here to describe it anew, especially as some of the fea- 
tures diene for the consideration of naturalists are not in 
accordance with the interpretation put upon them by previous 
observers. 
The body of this animalcule is colourless, but frequently has 
a slight yellowish or reddish tinge, which is derived by diffu- 
sion from the granular contents of the interior. The only 
legitimate colour present lies in the very faint red eye-spot (s). 
The form is variable, from elongate-ovate to ote with 
a gentle taper at the anterior third into a narrow truncate- 
emarginate head. Posteriorly the dorsal region is rounded ; 
but on the ventral face a broad triangular prolongation (/#), 
already spoken of as the homologue of the gubernaclum of the 
reptant Heteronemata, extends backward beyond the outline 
of the dorsum. The exact relation of this prolongation to the 
axis of the body is not to be determined beyond a doubt, be- 
cause of the constantly shifting attitude of the animal: at one 
moment the gubernaclum (fl?) ison the left, and then at the 
next instant it appears on the right of the mesial line, or fol- 
lows for awhile between these two points, according as the 
body keels over more or less from one side to the other or ba- 
lances itself in a median position. It appears most frequently, 
however, to be unilateral. 
The ameeboid contortions (fig. 46) of the body have already 
been mentioned; but I would add that this is only a resem- 
blance, a mere suggestion, if one may use the term, of the 
mode of locomotion of Ameba; for it is not, as in the latter, 
an actual flowing out of a glairy mass into protean reptant 
processes, but an exceedingly variable puckering, and always 
accompanied by a longitudinal contraction of the body, the 
one being evidently necessary to the other. If I may carry 
out the niceness of distinction further, I should say that, whilst 
Ameba is contractile and plastic, Astasia is retractile and 
flexible. 
The flagellum (fl) also, by its subterminal attachment to 
the head, carries out the typical plan of the reptant Hetero- 
nemata. It is based strictly on the ventral side of the front, 
descending from the latter with such an abrupt turn forward 
19* 
