696 



COELENTERATA~n. CNIDARIA. 



of the body, the digestion of food by the walla of the body cavity, the capture 

 of prey, and defence against enemies, by means of the formidable stinging 

 threads. 



The Hydra we have described is not an entirely stationary animal ; it can 



detach its disc and creep along by using alternately its tentacles and its disc, and 



sometimes even casts its body loose, using the expanded 



Hydroid Colonies disc as a float and the tentacles for swimming. Most other 



— Zoophytes. Hydroids, however, unite to form stationary colonies, the 

 young, which bud from each successive generation, re- 

 maining attached like branches, orby means of 

 rootlets, to the original parent animal. Such 

 colonies are often found encrusting empty 

 shells, which they sometimes entirely cover 

 with branched growths. One of them is re- 

 presented in Fig. 6. Some of the members 

 of this colony (A, A) bear a great re- 

 semblance to the Hydra, having the same 

 tube-like body, surrounded at the free end 

 by a crown of tentacles. But a large colony 

 of such highly contractile creatures as the 

 Hydra clustered together would be un- 

 manageable, 80, in these cases, their trunks 

 are generally enveloped in a more or less 

 stiff covering, which protects and supports 

 the colony. The free ends of the individuals, 

 with the mouths and tentacles, however, 

 protruding from the open ends of the en- 

 velopes, contract and expand freely. 



In the branched Hydroid figured, two 

 important points are to be noticed. First, 

 the gastral cavities of all the members of the 

 colony are in communication with each other, 

 so that, when one of them captures food, the superfluous nutritive fluid 

 produced can be passed on to the other members through the communicating 

 canals that run along the trunk and stems. Secondly, while most of the 

 members of a colony are Hydra-like, and help to feed the whole, others 

 (Fig. 6, B) are quite unlike the Hydra, differing both in the shape of the 

 body and in the character of the tentacles. These, which are called Medusoid 

 individuals on account of their resemblance to Medusee or Jelly-fish, are the 

 members in which the reproductive elements are produced. In some cases 

 the medusoid buds remain attached to the colony, and send forth ciliated 

 larvae, which swim away and settle later to start fresh colonies ; but, in the 

 colony depicted, the medusoid individuals break loose, swim about freely, 

 often greatly increasing in size, and after a time give rise to young forms 

 which attach themselves as Hydroids. 



Here, then, we have a most interesting phenomenon. These colonies of 

 Hydroids are stationary like trees or bushes, and, as in the case of trees or 

 bushes, it would be a disadvantage to them if their young grew up too close 

 around them; they would crowd each other to death. In the vegetable kingdom 

 there are many beautiful arrangements for securing the scattering of the 

 seed, but surely none more beautiful than the method adopted by some of 

 the Hyroids. The individuals selected to bear the young change their 



Fig. 6. — A HTDROn) Colont (Soujatii- 

 villea ra?nosa\ 



A, Hj dra like individuals, 



B, Medusoid individuals. 



