ZOOPHYTES 471 



simultaneously. Filling up the spaces between these large cells 

 are much smaller packing-cells with indistinct outlines, some of 

 which become transformed into the nettling-cells already noticed, 

 and force their way to the surface. Deep down in the ectoderm 

 are also to be found scattered nerve-cells of star-shaped outline, 

 collectively representing an extremely primitive form of nervous 

 system. 



The endoderm cells of Hydra are much larger than those of 

 the ectoderm, and are provided with muscular tails running in this 

 case transversely, so that collectively they make up a circular 

 muscle layer, by the contraction of which the body can be ex- 

 tended. These cells also have digestive functions, and their free 

 ends, which are directed towards the digestive cavity, can be 

 protruded into lobes by which food particles are bodily engulfed, 

 or, to speak more technically, ingested. Many of the cells are 

 also provided with groups of protoplasmic filaments, which execute 

 lashing movements by means of which the contents of the digestive 

 cavity are kept circulating. These are called flagella (L. fla- 

 gellum, a whip), and though they are in some respects allied to 

 cilia, differ from them in the relative complexity of their move- 

 ments, their larger size, and the fact that but a small number 

 of them are to be found on the same cell. In the green Hydra, 

 the endoderm cells contain numerous spheres in which the charac- 

 teristic pigment is contained, while in other species similar spheres 

 are present, differing, however, in the nature of the colouring 

 matter. 



From what has been said concerning Hydra it will be seen 

 that in this animal there is but little specialization or division of 

 the work of the body. As to digestive organs, the animal is little 

 more than an animated stomach, food being procured by the action 

 of the tentacles, and digested by the endoderm. Circulatory organs 

 are entirely absent, and, indeed, they are not necessary, for the 

 digested food can easily diffuse to all parts of the body. Waste 

 products are similarly easily got rid of, and there are therefore 

 no special organs of respiration or excretion. Both nervous system 

 and sense organs are in a very undifferentiated condition, the latter 

 being chiefly represented by sensitive " trigger hairs ", with one 

 of which each thread-cell is provided. 



Development (fig. 289). — New Hydrse are produced either by a 

 process of budding or from eggs. In the former case, a little knob 



