70 ATLAS OF BIOLOGY. 



iii. A younger example of the same, everted. 

 iv. The small fixed nematocysts. 



v. The resting and active conditions of a third variety. The filaments of these stain 

 very deeply with magenta. All drawn to same scale. F. 3. 



Hartog (87) has accounted for the occasional presence of thread-cells in the endoderm. 



FIG. X. Small portion of a transverse section across the hody of a green Hydra. 

 Picric acid, alcohol, borax carmine. 



The cilia are drawn from an osmic acid preparation. (See Parker 97.) D. 3. 



FIG. XI. Isolated examples of the chlorophyll-bearing bodies of the same. Teased 

 up fresh in water. Gundlach's j^th immersion. 



FIG. XII. A small portion of a similar section to Fig. X., with a piece of the support- 

 ing lamella, c.L, seen en face. 



Neither interstitial tissue nor endoderm cells are drawn. H. fusca. Osmic acid. D. 3. 



FIG. XIII. Larger cells of the ectoderm, isolated by bichromate of ammonia. F. 3. 

 FIG. XIV. Cells of the interstitial tissue, treated in the same manner.* 



FIG. XV. The greater portion of a solitary endoderm cell from H. fusca, isolated. 

 Picric acid, and borax carmine. F. 4. 



FIG. XVI. One of the sooty-particle bearing portions of the same. Gundlach's i x 6 th 

 immersion. 



Lankester (94) has suggested the most recent interpretation put upon these bodies, 

 and has finally set at rest the real nature of the chlorophyll-bearing bodies of H. viridis, 

 referred to above. The facts set forth in his paper have a most important bearing upon 

 the probable unity of the green and brown Hydra. 



FIG. XVII. A large brown Hydra, bearing at the same time asexually produced buds 

 and sexual organs. A. 2. 



FIG. XVIII. a testis, and XIX. an ovary of the above, at a later stage of develop- 

 ment. 



FIG. XX. Outline sketches of portions of two brown Hydra. 



The left one bore three ovaries, two of which are figured. The right-hand one bore 

 nine testes, of which four are figured. 



This drawing does not by any means represent the maximum development of the male 

 organs, but it suffices to show that the testes need not necessarily appear only at the bases 

 of the tentacles. 



* I am strongly of opinion that these often form syncytia. 



