and Urticating Cells. 251 
The richness of the South-Australian shore-fauna in Plu- 
mularide is extraordinary. I have seen about forty different 
forms, the skeletons of which are in part already described. 
I have examined most of them in the living state, and would 
here communicate some observations which I have made 
upon their guard-polyps. 
Notwithstanding the extraordinary variety which we meet 
with in their number and arrangement and in their form and 
size, they can all be referred to three fundamental forms, 
which often occur together upon the same stock :— 
1. Guard-animals with urticating capsules. . 
2. Guard-animals with adhesive cells. 
3. Guard-animals with urticating capsules and adhesive 
cells. 
1. Guard-Animals with Urticating Capsules. 
The guard-animals with urticating capsules occur chiefly 
upon the Plumu/aria-like Hydroids; they agree with the 
figure given by Hamann *. 
The solid endodermal axis consists of elements like those 
of the.tentacular axis, and extends to the middle of the guard- 
animal. The endoderm of the branch upon which the guard- 
polyp is seated passes directly into the endoderm of the latter. 
The cells which form the inner lining of the ccenosarcal tube 
of the branch are turbid and completely tilled with granules. 
The nearest endodermal cells of the axis of the guard-animal 
are also turbid and entirely filled with plasma. It is only at 
some distance from the branch that the axial cells begin to 
acquire more the form of chorda-cells, such as characterizes 
the cells of the tentacular axis; but the endodermal cells of 
the machopolyps, even in the distal part of the axis, are much 
more turbid and rich in plasma than the corresponding struc- 
tures in the tentacles. In the contracted state they are quite 
opaque. 
The ectoderm is highly developed, and consists of two 
layers, an epithelial and a subepithelial layer. The surface- 
cells appear cylindrical when the machopolyp is contracted ; 
on the contrary, they are depressed and flat when the guard- 
animal extends itself. These surface-cells are for the most 
_ part entirely filled with plasma; they occur upon the whole 
basal part of the guard-animal. Between them and the thin 
supporting lamella, which is closely applied to the endodermal 
cells, there is a layer of smooth muscular fibres, which do 
* «Der Organismus der Hydroidpolypen,’ pl, xxv. 
