and Urticating Cells. 253 
of which longitudinal muscles are applied. That the exten- 
sion does not take place so rapidly as in the tentacle is easily 
explained by the fact that the axial cells of the guard-polyp 
have not attained the same degree of differentiation as the 
chorda-cells of the tentacular axis. This is certainly to be 
accounted for by the fact that the tentacles are phylogeneti- 
eally much older structures than the machopolyps, and con- 
sequently their axial cells had a much longer time to adapt 
themselves to their new purpose than the endodermal cells 
of those polyps which became transformed into guard-animals. 
If we start from a Protohydra with hollow tentacles and 
consider its transformation into the guard-polyp of a Plumu- 
larid, we meet with a gradual suppression of the gastral 
space. First of all the tentacles become solid; then the sto- 
machal walls grow together. Centralization of the gastral 
space first of all in the person and then in the stock. 
The machopolyps are certainly employed as defensive 
weapons, as they do not retract themselves when the polyp- 
stock is touched; on the contrary, they protrude themselves 
from their chitinous calices when the branch upon which they 
are seated is separated from the stock. 
How far the guard-animals with urticating capsules in the 
nematophores ot many Plumularie also act offensively I could 
not ascertain; but these, as well as the machopolyps with 
adhesive cells, hereafter to be described, may be employed in 
the capture of prey. In all species of Plumularia that I have 
investigated the urticating capsules at the centrifugal ex- 
tremity of the guard-polyp remained always at the apex 
of the animal, and consequently accompanied it in all its 
movements. I must particularly insist upon this in opposi- 
tion to Allman’s statement®*. Allman concludes, from his 
observations upon certain Plumularide, that ‘ the clusters of 
thread-cells, when they exist, remain quite stationary, being 
never carried out with the sarcode in its pseudopodial pro- 
cesses.”’ I think that J can demonstrate that this observation 
relates to another fact than Allman supposes; and I am sure 
that in all cases that I have observed, when urticating cap- 
sules and no adhesive cells occurred upon the machopolyp, the 
urticating capsules were never left behind in the movements of 
the animal. 
2. Guard-Animals with Adhesive Cells. 
The animals which belong to this category occur prin- 
cipally in those nematophores of the species ot Aglaophenia 
* ‘ Monograph of the Gymnoblastic Hydroids, vol. i. p. 116. 
