ii COELENTERA TA 25 



are seen to remain adhering to them. The tentacles then 

 contract and bend inwards, carrying the food down to the 

 mouth, whence it is passed into the body -cavity to be 

 digested. The means by which the tentacles hold the prey 

 will appear when their detailed structure is investigated (see 

 p. 27). 



Microscopic ^ examining sections of the body or of a 



Structure tentacle, under a microscope, it can be seen that 



of the Body- the body is multicellular. Every part of the 



walL body -wall is built up of two distinct layers of 



cells, an outer layer of smaller regular cells, which may be 



distinguished as the skin cells (Fig. 11, s), and an inner layer 



of larger irregular cells lining the body-cavity. These latter, 



since they alone have to do with the digestion of the food, 



may be called the digestive cells (Fig. 11, di). 



Between these two layers of cells is a thin, non-cellular, 

 transparent, gelatinous layer, the mesogloea. In Hydra mridis 

 the two main layers are apparent even to the naked eye, for 

 the skin cells are colourless, whilst the inner digestive cells 

 are a bright green, so that the cells of the two layers stand 

 out in marked contrast. 



The skin cells are mainly conical in shape, and 

 are arranged regularly side by side, with much 

 smaller oval cells, the packing cells, filling the spaces between 

 their inner narrow ends. The inner ends of all the larger skin 

 cells are prolonged into narrow processes, which run just 

 below the skin, parallel to the long axis of the body. These 

 are known as the muscle processes ; for they have great power 

 of contracting and expanding, and by so doing they cause 

 those remarkable changes of shape in the body of the Hydra 

 which have been noticed. (Compare with the axial muscle- 

 fibre in Vorticella, p. 15.) Each separate skin cell consists 

 merely of a nucleated mass of protoplasm, but unlike the 

 Amoeba cell, each has acquired a definite shape, and its func- 

 tions are limited. By its sensitiveness to contact, each cell 

 acts as an organ of perception, giving warning to the organism 

 of the presence of some external stimulus ; then by virtue of 

 their special contractile processes, these cells act also as the 

 organs of motion of the body. Respiration also takes place 

 doubtless over the whole surface of the body. In the pro- 

 cesses of nutrition, however, these cells take no part. 



