v EXDODERM 303 



supposed to be nerve-cells, and to constitute a rudi- 

 mentary 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. 76, 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 

 remarkable for containing one or more vacuoles, some- 

 times so large as to reduce the protoplasm to a thin 

 superficial layer containing the nucleus (Fig. 76, 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 trans- 

 verse sections of a living Hydra, when the endoderm- 

 cells are seen to send out long blunt pseudopods (psd) 

 into the digestive cavity, and now and then to withdraw 

 the pseudopods and send out from one to three long, 

 delicate flagella (ft) . Thus the endoderm-cells of Hydra 

 illustrate in a very instructive manner the essential 

 similarity of flagella and pseudopods already referred 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 hypo- 

 stome, 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. 



In Hydra viridis the endoderm-cells (Fig. 76, D) 



