JELLY-FISH ; ETC. 
4S9 
These form the floors of the polyp-cavities. Only in the outermost layer of the 
stock is there life, the inner mass being composed of dead skeleton. In this 
living layer there is a close network of soft branching tubes, from which rise the 
small polyps, the bases of which are connected together by this network. The 
polyps lie in cup-like depressions, and, when undisturbed, project outward through 
the pores, retreating instantly at the slightest disturbance. The polyps, like the 
pores, are of two sorts. Those inhabiting the larger pores are short and thick, 
with four short tentacles, resembling stalked globules, surrounding a comparatively 
spacious mouth. The polyps protruding from the more numerous smaller pores, 
which surround the large ones, are much longer and have no mouths. Each of 
these ends in a simple knob, below which, at intervals, and generally alternately 
on one side and the other, short simple branches are given oft*. The central 
polyp remains quite still, but those which surround it are constantly in undulating 
motion, often bending down to the mouth of the central polyp, which they appear 
stock of Hydractinia on a shell of the whelk inhabited by a 
hermit-crab (nat. size). 
to be feeding. Here, again, there is division of labour in an animal colony, the 
larger central polyp provided with a mouth being the feeding individual, while 
the mouthless nutritive individuals catch the prey. The smaller polyps also 
probably defend the colony, being far better armed with stinging-capsules than 
the larger polyp. The knobs with which the tentacles end are stinging-batteries. 
The manner of reproduction of the Hydrocorallia—which are tropical animals and 
assist in building up the coral-islands—is not yet known. They grow upon rocks 
or dead corals, often covering the skeletons of sea-fans ( Gorgoniidce ), and are even 
found in the Bermudas on old bottles thrown into the sea. In the latter case the 
lower side of the stock is quite smooth as if polished, and reproduces exactly the 
surface of the glass with all its markings. 
There are two families of these Hydrocorallia, as they are called, namely, the 
Milleporidce and the Stylcisteridce. They are of great interest as illustrating the 
marvellous adaptability of living forms. While the true corals, which are polyp- 
colonies somewhat differently organised from these hydropolyps, secrete great 
masses of solid rock, we find two small families of minute hydropolyps also 
