24 THE POPULAR SCIENCE MONTHLY. 



around the mouth is a series or several series of tubular offsets, ranged 

 radially about the stem. The shape of these blossom-like zooids 

 varies in the different species. In some varieties they are unprotected, 

 while in others the tentacles may be withdrawn into a horny, cup- 

 shaped sheath. The number of tentacles varies witli the different 

 species. The plates of Tubularia indivisa and Hydra vulgaris show 

 the tentacles expanded. The other plates give, in the magnified por- 

 tions, only the chitinous sheath, into which the polyp has withdrawn 

 itself. 



In the Plumularians, a branch of the Sertularian group, curious 

 little cups of the horny sheath are developed. Unlike the cups which 

 contain the living flower, these extensions are filled with the sarcode, 

 or soft, gelatinous flesh of the animal. This sarcode, or protoplasmic 

 flesh, acts like the flesh of the . rhizopods and amoibge ; long filamen 

 tary processes are extended, just as the rhizopods improvise legs or 

 arms when they need them, till sometimes the horny sheath is invested 

 in this living gossamer. The function of these cups is not known. 

 Allman considers them as special zooids, whose morphological differ- 

 entiation from the other zooids is carried to an extreme. Hincks be- 

 lieves them to be a lower form of life, in organic union with the higher 

 zooids of the hydroid colony. 



The horny sheath, which is described by earlier writers as an excre- 

 tion, is by Allman considered to be rather the result of metamorphosis 

 of tissue. In many varieties the stem and branches of the creature 

 are slender, horny, and pipe-shaped, and the chitinous sheath is jointed 

 at regular intervals, the joint being a mere break in the continuity of 

 the chitine, not a movable hinge ; while the living pulp within forms 

 a continuous body, and is invested by its sheath as the pith of a 

 plant is invested by its stalk. 



The generative buds are caecal offshoots from the body, the repro- 

 ductive elements always developing between the inner and outer 

 membrane {see Fig. 2, d). They sometimes, after developmentj free 

 themselves from the parent stem, and lead a roving life as medusae. 

 In some cases the nutritive bud has its alimentary function suppressed, 

 and, though not itself sexual, it is henceforth destined to produce 

 sexual buds, either directly or through the medium of a non-sexual bud. 



There is, it may almost be said, no differentiation of organs among 

 the hydroids. In the adult form they possess no organs of sense, and 

 have no circulatory, respiratory, nor nervous systems. All the func- 

 tions of life are performed without the intervention of special organs. 

 Voluntary motion takes place without muscles, sensibility is present 

 without nerves, respiration is performed without lungs, and digestion 

 goes on without a true stomach. The sea-water which flows within 

 and about the creature bears to it the oxygen necessary to the main- 

 tenance of vital combustion, as well as the small living creatures 

 and comminuted organic matter which form its food. Like the sea- 



