II HYDRA 93 



colour is due, and in correlation with this the animal seeks the light 

 necessary to the functioning of the chlorophyll, while the brown Hydras 

 seek rather the shade. 



The movement of the Hydra from place to place may be a slow gliding 

 movement carried out by small pseudopodia pushed out by the ectoderm 

 cells of the base, or a more rapid movement in which the Hydra attaches 

 itself by similar pseudopodial extensions of the ectoderm cells of its 

 tentacles while it temporarily detaches its basal end from the substratum 

 to re-attach itself elsewhere. 



When living under favourable circumstances and well supplied with 

 food the Hydra multiplies actively by an asexual process of budding. 

 A little pocket-like outgrowth comes to project from the body, it increases 

 in size and gradually takes on the form of a small Hydra, a circle of 

 tentacles sprouting out from its end and a mouth perforation developing 

 between them. Finally it becomes constricted off at its base as an 

 independent Hydra just like the parent except that it is smaller in size. 

 When the budding process is very active a number of buds may be present 

 on the parent at one time, and the buds may have secondary buds 

 sprouting out from them so that there is formed a continuous mass of 

 Hydra individuals forming a sort of colony. Such a condition however 

 is only temporary and before long the mass separates into its constituent 

 individuals which proceed to lead a free independent existence. 



Budding is not the only method by which the Hydra is capable of 

 multiplying asexually. Occasionally — though very rarely — it may be 

 observed to reproduce by fission, the Hydra dividing lengthwise into two, 

 the process commencing at the oral end and slowly spreading downwards 

 towards the base. 



After a more or less prolonged period during which the multiplication 

 of the Hydra is entirely asexual there comes a time when gonads make 

 their appearance — ^the season differing in different species of Hydra. 

 Probably we may safely say that the appearance of the gonads is associated 

 with the onset of conditions in some way unfavourable to the life of the 

 particular species. 



The fully developed ovary forms a conspicuous rounded mass pro- 

 jecting from the body of the Hydra, its interior filled by the large spherical 

 egg or macrogamete. The testis forms a rather more pointed projection 

 of a whitish colour. When fully developed, examination with the high 

 power of the microscope shows a wild commotion going on in its interior, 

 due to the active movements of the microgametes. Eventually the wall 

 of the testis ruptures and the microgametes disperse through the water. 

 The overwhelming majority are wasted — this is a general characteristic 



