HISTOLOGY OF HYDRA 235 



tentacles and basal disc, present, but in scanty number, in 

 the proximal region of the body, abundant in the distal region, 

 and very abundant in the hypostome. Two forms of gland-cell 

 are shown in fig. 49, i^and G. Both have a wine-glass-shaped 

 body, the broader end turned towards the gastrovascular cavity, 

 the opposite end tapering to form a fine filament which runs 

 between the nutritive endoderm cells and is inserted by a 

 slight basal swelling on the mesoglcea. Near the thinner end 

 of the wine-glass-shaped body is a nucleus similar to but 

 smaller than the nuclei of the digestive cells. In the one kind 

 of gland-cell the body consists of dark finely-alveolar protoplasm 

 containing numerous minute granules and occasionally a few 

 larger globules. In the other kind, the free end of the cell — i.e. 

 the end nearest the gastrovascular cavity — consists of large 

 vacuoles separated by thin walls of transparent protoplasm, and 

 each vacuole is filled with a globule of highly-refringent 

 substance, the secretum of the cell. It is probable that the 

 finely-granular cells are simply transitional stages of the 

 coarsely-granular kind, which have discharged their secretion 

 and are forming a fresh supply. 



The tissues of a water-flea swallowed by Hydra are rapidly 

 dissolved by the action of the secretion poured out by the 

 gland-cells, and if a Hydra is killed whilst this process of 

 digestion is going on, the nutritive cells are seen to be loaded 

 with the products of digestion in the form of globules of a 

 proteid and fatty nature. There is also evidence that the 

 nutritive cells have the power of seizing solid particles by their 

 pseudopodia and ingesting them after the manner of an Amceba. 

 If finely-divided carmine is injected into the gastrovascular 

 cavity, the endoderm is soon afterwards found to be full of 

 minute granules of carmine, generally aggregated in the form 

 of little balls and enclosed in a vacuole. Very frequently, 

 also, nematocysts are found embedded in the endoderm cells. 

 Usually they are somewhat shrunken and altered in appearance 

 as if acted upon by the digestive secretions of the cells, and 

 three or four are often rolled together in a ball and contained 

 in a large vacuole. Most probably these nematocysts have 

 been swallowed with the food and ingested by the endoderm 

 cells, to be afterwards ejected with other indigestible matters. 

 There is no evidence that nematocysts are formed in the 

 endoderm. 



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