Dr. H. Fol on the Family Tintinnodea. 81 
faculty which makes up for their want of the power of turn- 
ing; they have the faculty of swimming backwards as fast 
and as readily as forwards; and, moreover, they can instan- 
taneously change the direction of their course. ‘The long pro- 
cesses with which they are provided, by striking against 
foreign bodies, warn the animal of danger, and enable it in 
good time to effect a precipitate retreat. This singular form 
is realized not only by the Zoée which I have just mentioned, 
but also by a new Infusorian of the family Tintinnodea. In 
fact this curious species has the habit of applying its test 
laterally against the cylindrical cells of an Alga furnished 
with long processes, which, although foreign to the animal, 
appear nevertheless to fulfil exactly the same functions as the 
processes of the carapace of the Zoée. Hach of the cells of 
this Alga bears a large process directed forwards, another 
directed backwards, and shorter lateral processes, between 
which the Infusorian attaches its test (Pl. IV. fig. 15). The 
number of Algal cells that the Tinténnus transports with it 
varies from one to four. 
The Tinéinn? swim with the aperture of the test in front, 
and have not at all the habit of moving in the opposite direc- 
tion; if they do so, it is only exceptionally and for a very 
short time. Our species, on the contrary, swims just as readily 
in one direction as in the other; and when the anterior point 
of the Alga meets with a foreign body, the animalcule takes to 
flight backwards as quickly as it had advanced. 
Another example of convergence of types by adaptation is 
furnished by an Infusorian the description of which I have 
been unable to find in any author, although the species is not 
rare. ‘The aborescent colonies of this Vorticellidan float in the 
sea; and if one touches them, they contract after the fashion 
of a Medusa, producing a movement of propulsion of the 
whole colony. 
Classification.—In the still imperfect state of our know- 
ledge of this group I think it most prudent to continue to take 
the characters of the test as the basis of classification ; and I 
consider myself the more authorized to act thus because the 
differences of the anatomical characters appeared to me to 
coincide with the sections that we obtain by taking into account 
only the test. 
Genus I. Trnrinnus, Schrank. 
. e 2 ee 
Diagn. emend. Vest smooth, firm, chitinous, transparent, 
composed of two lamella united by septa which are not very 
