156 AN INTRODUCTION TO ZOOLOGY 



enzymes allied to trypsin. As a result of this activity the prey 

 disintegrates, and as the small pieces of it float about in the cavity 

 they are seized by the epithelio-muscular cells and engulfed in the 

 same manner as Amoeba ingests its food. They are then digested in 

 the cell. Thus we find two distinct types of digestion within the 

 body of Hydra ; firstly, intercellular digestion in a cell-lined gastral 

 cavity, the only kind met with in the higher metazoa ; and secondly, 

 the intracellular digestion, in which the food is broken up within the 

 cell itself as in the Protozoa. The only phenomenon allied to this 

 latter type in the higher animals is that we have already dealt with 

 in the case of the white blood corpuscles of Rana, termed phagocytes, 

 for in this case the bacteria are swallowed and eaten by the cells. 

 Any insoluble residue, including starch, for apparently Hydra does 

 not secrete a starch-splitting enzyme, is voided through the mouth, 

 which therefore functions also as an anus. 



Another phenomenon calls for notice here, and that is the 

 relation between the tiny green plants and the entoderm cells. 

 These green algae, when they are included in an animal cell, as they 

 are in certain protozoa and worms, are termed in general Zoo- 

 chlorellae, and the particular species particularly associated with 

 H. viridis is Chlorella vulgaris. They are in no sense parasites, for 

 they are not detrimental either to the structure or function of Hydra ; 

 in fact they are quite the reverse. The animal not only lodges and 

 protects the algae, but produces as waste products carbonic acid gas 

 and various nitrogenous matters which serve the plant as food. 

 On the other hand, the plant removes these waste materials for the 

 animal, and in its turn makes Oxygen, which is of use to Hydra, and 

 perhaps also certain carbohydrates. Certain it is that a H. viridis 

 freed of its Chlorellcz does not live so vigorously as with them. Thus 

 we have a living together which results in a mutual benefit to both 

 parties concerned, and this, which we term Symbiosis, is to be sharply 

 distinguished from parasitism. 



Respiration and excretion in Hydra other than that just dealt 

 with appear to be carried out as in Amoeba, by diffusion over the 

 general surface of the body, and there is no special organ for the 

 performance of either function. 



Reproduction in Hydra takes place, as we have seen, by 

 two methods, by budding and by the production of gametes. In 

 asexual multiplication there is first of all an increase in the inter- 

 stitial cells, and then the two layers of the body wall grow out into 

 a tiny knob-like projection whose internal cavity is continuous with 

 that of the parent. At first it is rounded, but a series of tiny sprouts 

 at the distal extremity mark the beginning of the tentacles, which 

 soon grow. In the middle of these appears an opening, the mouth, 



