382 ZOOLOGY. 



tophores are not parts of the Hydra, but independent 

 organisms unicellular Algae living in close and mutually 

 advantageous association (symbiosis) with it. But as the 

 chromatophores have neither nucleus nor cell-wall, they 

 cannot be unicellular Algae. 



In H. fusca, chromatophores are also found, but with another 

 colouring-material instead of chlorophyll. It has been suggested 

 that the two species are really the same under different conditions 

 which cause the one or the other colouring material to appear. But 

 there is no evidence of this : the two forms live side by side under 

 the same conditions ; and colour is not the only difference between 

 them. IT. fusca is always longer and slenderer than H. viridis, and 

 has much longer and finer tentacles. The balance of evidence is in 

 favour of their being distinct species, though the reader who 

 remembers what we have said in chapter xii. will see that there is 

 no absolute criterion. 



9. Physiological Differentiation. Taking the two 

 layers as a whole, we see that the ectoderm is essentially 

 protective, sensory, and nervous, the endoderm digestive. 

 Both are also in some degree contractile (or muscular) and 

 in places secretory. Both are epithelia, though in both 

 we have, as it were, a beginning of differentiation of muscle- 

 tissue, and in the ectoderm of nerve-tissue. The secreted 

 mesoglcea evidently represents a very simple kind of 

 endoskeleton, but there are no connective tissues of any 

 kind. Neither is there any rudiment of a vascular system, 

 the products of digestion and of katabolism easily diffusing 

 from cell to cell in so simple an animal. The absence of 

 respiratory organs is not remarkable, for in so small an 

 animal every cell is in contact with the water, or nearly so. 

 It is somewhat surprising, however, to find no trace of 

 renal -excretory organs, seeing that even Amoeba has a 

 contractile vacuole for this purpose. Possibly waste- 

 products may be excreted along with the useful products 

 by the gland-cells of the endoderm. The absence of true 

 cilia in Hydra is accounted for by its method of feeding ; 

 cilia are abundant in some allied organisms. 



10. Asexual Keproduction. So far we have said 

 nothing about reproduction, for Hydra stands alone among 



