160 AN INTRODUCTION TO ZOOLOGY 



as plant-animals, Zoophytes. Quite small, as a general rule, these 

 organisms nevertheless when viewed alive under even a low 

 magnification form some of the most beautiful objects to be met 

 with in the animal kingdom. Closely adherent to the seaweed frond 

 will be found a network of fine threads which constitute a kind of 

 root, the hydrorhiza, serving for attachment. From this arise a 

 number of thread-like stems, the hydrocauli, which on closer examina- 

 tion are seen to give off a number of small lateral branches, each one 

 terminating in a tiny knob. Even a low magnification will reveal 

 the fact that each of the small lumps is in reality a tiny flower-like 

 hydriform being, closing in response to the slightest touch when 

 living, and variously termed a hydranth, a polyp or a zooid. Thus 

 we have a compound organism composed of a large number of 

 hydroid individuals united together by common stems and roots, and 

 such an assemblage we call a colony or stock. We saw that in Hydra 

 sometimes the individual could bud off daughters, and that before 

 these were shed they could in themselves give rise to buds producing 

 a sort of temporary colony, and the stock in Obelia is formed much 

 in the same way. A single hydriform being gives rise to a bud 

 which grows upwards, turning the head of the parent to one side 

 and also produces a bud, and this in its turn another bud, and so on. 

 In this way we have produced a long stem, the end of which is always 

 growing, and in Obelia, as the buds are produced on alternate sides, 

 we have formed a characteristic zigzag stem with zooids borne 

 on short stalks at its angles. From their mode of origin, too, it will 

 be seen that the gastral cavities of all the hydroids will remain in 

 continuity with one another by means of the hollow stem. The 

 hydrorhiza is also a series of tubes, so that the whole colony, some- 

 times consisting of an enormous number of individuals, has a series 

 of gastral cavities intercommunicating by means of gastro-vascular 

 canals. 



In order to keep the stem erect we find that the colony secretes 

 around itself a supporting and protective exoskeleton composed of a 

 horn-like substance, chitin. We distinguish the skeletal substance 

 as the perisarc as opposed to the ccenosarc, i.e. the living parts which 

 it surrounds. In the regions of the hydroids the perisarc expands 

 into cup-shaped receptacles, the hydrothecae, into which the polyps 

 can, and do, completely withdraw when disturbed. Then, too, 

 the perisarc of the stalk joining the hydranth to the main axis 

 exhibits a series of ringlike constrictions or annulations, as may also 

 the main stem itself at these points. The number and arrangement 

 of these differs in various species. 



The particular type of individual noticed above is almost 

 exclusively concerned with the procuring, digestion and distribution 



