JELLY-FISH, ETC. 



489 



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 off. 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 OP 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 wliich 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 (Gwgoniidw), 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 Stylasteridce. 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 



