298 HYDRA CHAP. 



with large nuclei occur (Fig. 75, c, nv. c) ; they are supposed 

 to be nerve-cells, and to constitute a rudimentary nervous 

 system (compare p. 167). 



The ectoderm cells of the foot differ from those of the rest 

 of the body in being very granular (Fig. 74, a). The 

 granules are probably the material of the adhesive secretion 

 by which the Hydra fixes itself, and these cells are therefore 

 glandular (p. 130). 



The endoderm consists for the most part of large cells 

 which exceed in size those of the ectoderm, and are re- 

 markable for containing one or more vacuoles, sometimes 

 so large as to reduce the protoplasm to a thin superficial 

 layer containing the nucleus (Fig. 74, a and b, end). Then 

 again, their form is extremely variable, their free or inner 

 ends undergoing continual changes of form. This can be 

 easily made out by cutting transverse sections of a living 

 Hydra, when the endoderm cells are seen to send out long 

 blunt pseudopods (j>sd) into the digestive cavity, and now 

 and then to withdraw the pseudopods and send out from 

 one to three long, delicate flagella (JP). Thus the endoderm 

 cells of Hydra illustrate in a very instructive manner the 

 essential similarity of flagella and pseudopods already re- 

 ferred to (p. 250). In the hypostome the endoderm is thrown 

 into longitudinal folds, so as to allow of the dilatation of 

 the mouth in swallowing. 



Amongst the ordinary endoderm cells are found long 

 narrow cells of an extremely granular character. They are 

 specially abundant in the distal part of the body, beneath 

 the origins of the tentacles, and in the hypostome, but are 

 absent in the tentacles and in the foot. There is no doubt 

 that they are gland-cells, their secretion being a fluid used 

 to aid in the digestion of the food. 



