9 a ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



r bodies (see below, p. 185) constitutes the macro- 



derm of the Hydra, even in the living specimen examined 



mder a low power of the microscope, shows a striking difference 



:-m in that it is distinctly coloured, green or brownish 



to the species, whereas the ectoderm is colourless. The 



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



:>iderably larger than those of the ectoderm and 



11 them also in other details. The muscular tails are arranged 



iinally but circularly, so that when they contract they cause 



Ira to assume an attenuated threadlike form, increasing greatly 

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



'arries flagella (Fig. 34, B) which by their constant lashing 



"il and other particles to be swirled about in the fluid of the 



. In a Hydra which has been starved the endoderm cells 



.Holes. In a well-fed specimen on the other hand these 



onspicuous while there occur scattered about in the protoplasm 



< ply staining protein-spheres composed of stored-up food 



d while there are also present clumps or isolated granules of 



brown excretory substance. 



Amongst the ordinary endoderm cells there occur, especially in the 



i" t In- oral cone, occasional gland cells squat-shaped cells, without 

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



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



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



ralvsed by the nematocysts of the tentacles is drawn to the mouth, 



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



' <T its digestible portions gradually disintegrate under the influence 



ferments passed into the cavity by the gland cells. Here 



>s of digestion taking place not within the substance of a 



ell (mtracellular). as was the case in the Protozoa, but in a space bounded 



' ' T< -ellular), the dissolved products of digestion being absorbed 



'IK bounding the space. The process of digestion throughout 



i- for the most part intercellular. In the case of Hydra 



till persists a certain amount of intracellular digestion, 



disintegrated food are ingested bodily by the inner ends 



i<l"derm cells and their digestion completed within the cytoplasm 



'II. 



case of the -rim Hydra the endoderm is infested with numerous 

 <!>hyll-comaining Flagellates to which the bright green 



