vm. D, 5 Light: Morphology of Eudendrium 345 



This thickness is only found in the shortened and widened fertile 

 female hydranths, that in the normal hydranth being about 30 

 fx/j. These cells have the characteristic "dunklere Farbung" of 

 the ectoderm of the base of hydra. 



The endoderm of the ccenosarc consists of rather large cells 

 with well-defined cell walls. An average cell measures 31 ixfi 

 in length, 12 n/t in breadth, and 21 mx. in thickness. The nuclei 

 are more easily distinguishable than are those of the ectoderm. 

 They are small, spherical in shape, with no definite chromatin 

 figure excepting a more or less constant nucleolus which stains 

 a blackish tint in hsematoxylin. They are found near the distal 

 wall of the cell. The cytoplasm is much denser than that of 

 the hydranth endoderm, and contains, toward the distal end, 

 thickened granular protoplasm which takes a deep stain. Near 

 the upper end of the pedicel the cells are crowded together and 

 consequently elongated (fig. 8). Zooxanthellse are not so nu- 

 merous here as in the endoderm of the 

 hydranth, many cells containing none, others 

 from 1 to 3. 



Hydranth. — The ectoderm of the lower 

 part of the sense ring has already been 

 discussed under the ccenosarc. It is sepa- 

 rated from the rest of the hydranth by the 

 groove of the sense ring. The ectoderm of 

 the hydranth above the sense ring is much p,^ 3 cross section of a 

 the same throughout, differing mainly in the pedicel just beiow tha 

 size of the cells. The cells are flattened, hydranth in B«<fe„dr.„« 

 rough, and irregular, stain deeply, and con- 

 tain a considerable amount of granular material. The nuclei are 

 small and easily distinguishable. The ectoderm, although thin, 

 is distinct on the body of the hydranth below the tentacles, but 

 it thins out rapidly on the base of the tentacles and is almost 

 undistinguishable in many places on their distal portions. 



The ectoderm of the tentacles seems to consist of a very thin 

 layer of protoplasm lying between the mesogloea which has here 

 become very thin and an outer continuous wall corresponding to 

 the cell walls of more normal ectoderm. There seems to be little 

 if any separation of this layer into individual cells, and nuclei 

 cannot be distinguished. The nematocysts cause decided swell- 

 ings of this thin layer of ectoderm (Plate I, fig. 2). It is 

 worth noting that the ectoderm of all sides of the tentacles is of 

 the same thickness, not being, as in so many hydroids, thick 

 on one side of the tentacle and thin on the other. So thin is 

 this ectodermal layer and so close together are the mesogloeal 



