No. 394.] REVIEWS OF RECENT LITERATURE. 835 
may, whatever its imperfections in detail, be truly called a natural 
one. In the successive examination of the structures above named, 
the fact is clearly brought out — a fact more or less patent to all who 
have worked extensively on the Rotifera — that all the widely sepa- 
rated members of the group are connected by transitional forms with 
the worm-like Notommatidz. The latter form thus a central group, 
from which the others have diverged along different lines. Certain 
of the Notommatide present what must be looked upon as the prim- 
itive form of the structures in question — a soft cuticula ; ciliary 
organ, consisting of a flat disk on the ventral side, covered with 
undifferentiated cilia ; forcipate mouth parts, and two pairs of lateral 
sense organs. From this central group some six or seven lines of 
evolution in different directions are traced, each marked by succes- 
sive changes from the primitive form of the organs above mentioned, 
in correspondence with the life habits of the animals, 
These studies form the basis of the classification which follows. 
In this classification one or two points are worthy of especial men- 
tion. The division of the larger part of the Rotifera on the basis of 
the stiffness or softness of the cuticula into the two general groups, 
Loricata and Il-loricata, which has proved such a stumbling-block to 
a natural classification, is done away with. To the presence or 
absence of the foot little importance is attached. Whether one can 
or cannot agree in detail with the exact arrangement of the genera 
within the system as given, I believe it must be admitted that this is 
. the nearest to a natural system of the group that has ever been given, 
and that it will be upon some such lines as these that the final clas- 
sification of the Rotifera will be made. 
The paper is one deserving of study by all students of the Rotifera. 
It is, moreover, a model for the way in which the general life relations 
of animals may be brought into correlation with their morphology 
and classification ; as such it has claims on the interest of others 
beside the specialist in Rotifera. Unfortunately, the work is ren- 
dered somewhat less easily accessible in that it is written in the 
Danish language instead of in one of the four languages that form 
the recognized necessary linguistic equipment of the man of science. 
H. S. JENNINGS. 
Segmentation of Insect Head. — The segmentation of the insect 
head, as seen in the Collembola, has been studied by J. W. Folsom.’ 
1 Folsom, J. W. The Segmentation of the Insect Head, Psyche, vol. viii, 
pp. 391-394- August, 1899. 
