92 ZOOLOGY FOR MEDICAL STUDENTS chap. 



formation of two polar bodies (see below, p. 185) constitutes the macro- 

 gamete. 



The endoderm of the Hydra, even in the living specimen examined 

 as a whole under a low power of the microscope, shows a striking difference 

 from the ectoderm in that it is distinctly coloured, green or brownish 

 according to the species, whereas the ectoderm is colourless. The 

 endoderm (Fig. 33, A, end) is composed mainly of a layer of myoepithelial 

 cells (Fig. 34, B) considerably larger than those of the ectoderm and 

 difiering from them also in other details.. The muscular tails are arranged 

 not longitudinally but circularly, so that when they contract they cause 

 the Hydra to assume an attenuated threadlike form, increasing greatly 

 in length. The end of the cell next the coelenteron is rounded and in 

 many cases carries flagella (Fig. 34, B) which by their constant lashing 

 cause food and other particles to be swirled about in the fluid of the 

 coelenteron. In a Hydra which has been starved the endoderm cells 

 contain large vacuoles. In a well-fed specimen on the other hand these 

 are inconspicuous while there occur scattered about in the protoplasm 

 numerous deeply staining protein-spheres — composed of stored-up food 

 material — while there are also present clumps or isolated granules of 

 brown excretory substance. 



Amongst the ordinary endoderm cells there occur, especially in the 

 region of the oral cone, occasional gland cells — squat-shaped cells, without 

 muscular tails, and containing droplets of secretion in their cytoplasm. 



As regards the physiology of the Hydra we have to notice first its 

 process of feeding. A small food organism such as a Water-flea caught 

 and paralysed by the nematocysts of the tentacles is drawn to the mouth, 

 which opens to receive it, and slowly passed into the coelenteron. In 

 the latter its digestible portions gradually disintegrate under the influence 

 of digestive ferments passed into the cavity by the gland cells. Here 

 we have a process of digestion taking place not within the substance of a 

 cell (intracellular), as was the case in the Protozoa, but in a space bounded 

 by cells (intercellular), the dissolved products of digestion being absorbed 

 by the cells bounding the space. The process of digestion throughout 

 the Metazoa is for the most part intercellular. In the case of Hydra 

 however there still persists a certain amount of intracellular digestion, 

 for fragments of disintegrated food are ingested bodily by the inner ends 

 of the endoderm cells and their digestion completed within the cytoplasm 

 of the cell. 



In the case of the green Hydra the endoderm is infested with numerous 

 sjrmbiotic chlorophyll-containing Flagellates to which the bright green 



