52 COELENTERATA [CH. 



clear one, termed the ectoderm (Gr. CKTOS, external; Sepjua, skin), 

 and an inner one called the endoderm (Gr. IvSov, inside), which 

 is green in Hydra viridis and brownish in Hydra fusca-, so 

 that we may speak of a skin as distinct from the lining of the 

 coelenteron (Figs. 17 and 18). It is further possible to make out 

 under the microscope that at any rate the outer layer is not 

 homogeneous, but is - composed of separate small pieces. It is 

 necessary, however, to examine thin sections of specimens which 

 have been hardened by being soaked in corrosive sublimate or some 

 similar reagent, before one can really get a good idea of the 

 structure of the "skin" and of the "inner lining" of the polyp. 

 Then it is seen that both are made up of the repetition of similar 

 parts, and that in each of these parts there is a single nucleus. 

 Such a portion of protoplasm, marked off from the surrounding 

 parts by a definite boundary, is called by zoologists a cell. The 

 wall or boundary of the cell probably consists of a thin layer of 

 some secretion, in many cases, if not in most, traversed by bars or 

 sheets of protoplasm connecting the cell with its neighbours. 

 Around this term "cell" many battles have been waged and its 

 t indiscriminate use has led to much misconception. 



It used to be said, for instance, that the Coelenterata 

 were multicellular animals, as opposed to the Protozoa, which 

 were unicellular. JNow it has already been pointed out that the 

 centre of the vital processes is the nucleus, which controls the 

 processes going on in the protoplasm, and that in some of the 

 Protozoa, such as Actinospfyaerium and Opalina, this essential organ 

 is repeated several hundred times. But an Actinosphaerlum or an 

 Opalina certainly does not correspond to a so-called cell of Hydra, 

 with its single nucleus ; the relation between them may rather be 

 define'd by saying that, whereas in Actinosphaerium the areas of 

 control of the various nuclei are not visibly delimited from each 

 other, in Hydra, on the other hand, this delimitation has to some 

 extent taken place, leading to the appearance of cell-structure. 

 But not only are cells to be detected in Hydra ; the cells are not 

 all of 'the same kind. Those forming the endoderm are very 

 big and often have great watery vacuoles near their inner ends; 

 they also contain the coloured granules to which the colour of the 

 animal is due. The cells of the ectoderm or outer skin, on the 

 contrary, are much shorter than those of the endoderm and are 

 more or less pear-shaped, the broader end being turned out. Be- 

 tween their narrower bases we find groups of very small round cells 



