42 HYDROIDS AND JELLY-FISH 



of the two cell layers, each fragment is capable of developing into 

 a complete animal ; so that really the little creature is very appro- 

 priately named Hydra, after the monster of the old Greek fable. 



In general structure the Hydra is a simple sac, the walls of 

 which are composed of an outer or ectoderm layer one or more 

 cells deep, and an inner or endoderm layer one cell deep, the two 

 layers being separated by a thin, structureless middle lamella or 

 mesoderm. The inner layer or endoderm lines the whole inner 

 cavity of the sac-like body and the interior of the tentacles, and 

 is concerned in the digestion of food, which is taken in at the 

 single opening or mouth in the centre of the circle of tentacles, the 

 insoluble portions being ejected later through the same aperture. 

 Such is fundamentally the structure of all the Hydroida. 



We have seen that the Hydra forms buds upon its sides, 

 and that these ultimately drop off. Now, supposing that the 

 Hydra were to grow much larger, form a horny protective cover- 

 ing to its outer surface, produce numerous buds which, instead of 

 separating, elongated and in turn gave rise to other buds, all 

 remaining connected together, each bud crowned with its waving 

 circle of tentacles a plant -like colony would result. In fact, we 

 should have a typical Hydroid colony, such as may be seen grow- 

 ing in the deep rock-pools, and whose graceful, feathery, horny 

 textured branches are familiar treasures of the seashore, under 

 the name of Zoophytes l or Sea-firs. 



Such a typical Hydroid colony is formed by the pretty little 

 Obelia, to be found growing in branching filaments upon the 

 submerged wooden piles of piers and breakwaters, and also upon 

 the fronds of the wrack seaweeds. Under the microscope part of a 

 living colony presents a very striking and beautiful appearance, 

 every branch bearing numerous tentacle-crowned polyps, all vitally 

 connected with each other by the common living tissue, which 

 looks like a central pith inside the stem. Both stem and branches 

 are encased by a horny outer cover, and each little polyp is enclosed 

 in a glassy cup-like receptacle or hydrotheca. When the polyps 

 are fully expanded, they give to the colony the appearance of 

 some exquisite miniature tree that is covered with tinted, slender- 

 petalled, ray-like flowers. 



1 Greek, zoon, animal ; phyton, plant : the name originally given to these com- 

 pound Hydrozoa on account of their plant -like habit and appearance. 



