468 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
possible that the pelagic and free state are but phases in the course 
of its existence. Its form is best understood by speaking of it as 
a regular octohedron, formed of two elongated pyramids with square 
base, and all the angles rounded. When alive, however, the creature 
is so contractile in every direction that it presents the most varied 
forms. 
The supporting lamella which separates the endoderm from the 
ectoderm has been pretty satisfactorily described by Claus ; the author 
gives his own account of the parts of which it is composed. For the 
greater part of the body the ectoderm consists of a single layer of thick 
cells with very short cilia ; they have no muscular processes connected 
with them, and it is to their own contractility that we must ascribe the 
contractility of the body. Among the large ectodermal cells are the 
glandular cells and the cnidoblasts. The former are easily divisible 
into two parts — the true cell with its nucleus, and the gland, which is 
quite peripheral and has a very small excretory orifice. Claus did 
not distinguish the glandular cells from the vibratile cells which cover 
them. Some of the cnidoblasts are small and almost spherical ; others, 
two and a half or three times as large, are still more spherical than oval. 
The former are found all over the surface of the body, while the larger 
are limited to a few placed on the median lines of the surfaces of the 
aboral pyramid, and are found in large number on the longitudinal 
ridges. After a lengthened description on the action of stinging cells, 
the author passes to an account of the sense-organs, on which various 
authors have expressed their opinions, but the mode of action of these 
bodies still remains a matter for conjecture. 
While venturing to criticize some of Claus’ expressions, the author 
can only “imitate his wise reserve as to the definite position which 
Tetrajplatia ought to occupy in our classifications.” 
Histology of Hydra.* — Herr K. C. Schneider has investigated the 
histology of Hydra fusca with special reference to the nervous system of 
Hydropolyps. In the ectoderm we may distinguish epithelial and sub- 
epithelial cells according to their position ; the epithelio-muscular and 
stinging cells are epithelial ; the former are divided into investing and 
secreting cells, while the latter are only a modification of the former. 
The investing cells possess a cuticle, the peculiar property of which is 
indicated by the fact that the very delicate alternate with thicker areas ; 
stinging cells are deposited in them, and they multiply by indirect 
division. The secreting cells give off granules which are arranged in 
parallel cords on the protoplasm ; they contain no stinging cells, and 
their mode of multiplication has not been observed. All the muscle- 
cells give off basally long contractile fibres which are invested by proto- 
plasm for their whole length; the direction of the fibres is longitu- 
dinal, but they seem to be somewhat sunk into the supporting lamella, 
into which they send processes. The stinging cells only reach the 
surface by means of the cnidocils which traverse the cuticle of the 
covering cells. Centrally they contain a stinging capsule, around which 
in most (and perhaps all) cases there is a muscular layer ; this often, 
especially in the tentacles, passes into a muscular tubular stalk, within 
* Arch. f. Mikr. Anat., xxxv. (1890) pp. 821-79. 
