IQI5.| J. RircH1e: Hydroids of the Indian Museum. 545 
sented by a solid endoderm, an exceedingly thin mesogloea, and 
an ectoderm of greatly varying thickness. 
The solid endoderm is composed of many thin-walled cells, 
with sparse protoplasmic content which often simply lines the cell- 
wall and includes a small oval nucleus. The cells appear to be 
arranged, but somewhat irregularly, in four radial series of hexa- 
gonal cells, the bases of which rest upon the mesogloea, while the 
pyramids which form their apical regions interlock towards the 
centre of the tentacle. A longitudinal median section of a ten- 
tacle, therefore, generally exhibits a series of lateral walls of endo- 
derm cells at right angles to the mesogloea, and in the centre a 
zigzag line representing the junctions of the pyramidal apices. 
Both in the character of its cells and in their arrangement the 
solid endoderm of this form differs very markedly from the solid 
endoderm of general occurrence in the tentacles of hydroids. In- 
stead of thick-walled (‘‘ notochordal’’) cells arranged with great 
regularity in a single series lying along the long axis of the 
tentacle, as is the general rule, there are here delicate, thin-walled, 
multiserial cells. 
The mesogloea of the tentacle-cells calls for no remark except 
that it is of extreme tenuity scarcely exceeding Iu in thickness 
throughout the whole length of the tentacle. 
The ectoderm of the tentacles falls into two distinct zones, 
the ring-like or globe-like swellings, which I shall designate nodes, 
and the spaces between them (see pl. xxxa, figs. 7 and 8). Inthe 
inter-nodes or inter-anuular zones the ectoderm, even when the 
tentacle, in contraction, is at its stoutest, consists of a very thin 
layer of much flattened epithelial cells. In an extended tentacle 
this layer owing to its tenuity becomes scarcely visible. A rare 
cnidoblast, similar to the lesser variety in the nodes, forms the 
only inclusion in the internodal cells. 
The nodes are composed of a zone of large oval cushion- 
shaped cells, closely appressed to each other laterally in a single 
row. Occasionally, however, incomplete zones or isolated indivi- 
duals of these cushion-shaped cells occur in the inter-nodal areas. 
At the junction of nodes and inter-nodes the internodal ectoderm 
conforms to the outline of the nodal cells, being banked up against 
their curved walls with a gentle slope. The size of the nodal 
cells varies with the contraction of the tentacle, but the short 
diameter (parallel to the long axis of the tentacle) usually lies 
between 15» and 27,, while the height varies from I2, to 22n. 
Apart from inclusions the nodal cells contain little cytoplasm, the 
greater part of their interior being occupied by a large vacuolar 
space. Upon the base of the cell, however, there lies a thin layer 
of cytoplasm, and a median nucleus, 5 by 3 in diameter, contain- 
ing a small nucleolus and surrounded by a sparse coat of cyto- 
plasm whence delicate strands radiate outwards. The whole struc- 
ture of the cell appears to be organised in relation to its function 
as a battery cell. I shall, therefore, discuss here the arrangement 
and structure of its cnidoblasts. 
