THE JELLYFISH AND THEIR ALLIES 3463 



often delicately branching integumentary skeleton usually found in the hydroid- 

 polyp stock, that of the Hydrocorallia contains ninety-seven per cent, of carbonate 

 of lime, and forms rough, solid-looking masses, with lobed processes or bosses like 

 those figured on p. 3464, or else (Stylasteridce) branches, like the precious coral of 

 commerce. The whole surface can be seen even with the naked eye, but still better 

 with a lens, to be covered with small pore-like apertures. Closer examination shows 

 that these are of two sizes, a larger central pore being surrounded by an irregular circle 

 of from five to eight smaller ones. The mass of the colony is traversed by an irregular 

 system of numerous branching canals of different sizes. In vertical sections, 

 indistinct layers can be seen running almost parallel with the outer surface. 

 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 by this network. The polyps 



STOCK OF Hydractinia ON A SHELL OF THE WHELK INHABITED BY A HERMIT CRAB. 



(Natural size.) 



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 

 to be feeding. Here, again, there is division of labor 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 man- 



