THE ORTHONECTIDA. 
229 
that is to say, organisms composed fundamentally by two 
layers of cells, the one within the other — an ectoderm or 
external layer of cells, which are mostly ciliated, and an 
endoderm constituted by larger cells,’which are more granular, 
and form the lining of a central cavity devoid of either mouth 
or anus. 
The first ring terminates in a blunt cone anteriorly, which 
carries a bunch of stiff cilia. It is followed by a cylin- 
drical ring of the same length, the whole surface of which is 
beset with papillae, which are disposed in eight or ten longi- 
tudinal lines and in four or five transverse series ; this ring 
is the sole part of the body which does not present vibratile 
cilia. 
The third ring is larger by itself than the two first 
together ; it gradually widens towards its posterior ex- 
tremity. 
The fourth metamere is of the same dimensions as the 
papilliferous ring; it is followed by a terminal ring provided 
with longer cilia, forming a brush at its posterior extremity. 
This last ring is conical and subdivided into two metameres, 
less distinct than those in front. 
The last rings form a sort of club, with which the animal 
strikes the water, independently of the movement of the 
cilia, giving sharp strokes, which one is immediately led to 
attribute to the contraction of muscular elements. We 
shall point out below where these elements occur. 
Such is the elongated variety. The ovoid variety differs 
from it only by being a third less in length and a greater 
breadth. It seems, at first sight, as though it had been 
derived from the former by a contraction along the long 
axis ; but it is easy to assure one^s self that this is not the 
case, and that it is, on the contrary, the elongated form 
which is the ultimate condition of the ovoid form, which 
latter is only the final embryonic phase of the animal. 
For a more complete study of the anatomy of Rhopalura 
it is necessary to employ very high magnifying power, the 
objective 6 of Verick,or better, the objectif 9 (immersion) of 
Hartnack. 
The ectodermal cells then appear very distinctly, except 
in the papilliferous zone where it is very difficult to deter- 
mine their number and form. All the other metameres are 
made up by a single transverse row of cells, and the differ- 
ence in the length of the metameres depends entirely on the 
difference in the length of the cellular elements which com- 
pose it. The terminal rings are formed entirely by four 
cells, as in the Dicyemida, the median rings comprise six or 
