328 DESCRIPTION OF THE PLATES. 



the body and bringing a tentacle into contact with the surface upon 

 which it rests, then detaching the disc, and bringing it up to the 

 tentacle and reattaching it. 



Hydra rarely occurs hermaphrodite as figured here, but one and the same 

 individual may produce both testes and ovaria at different times. These structures 

 are organs in Hydra, not zooids. This fact is proved (i) by their development out 

 of interstitial ectoderm cells, as in the organs of many sexual Medusae, and (2) by 

 the absence of any evagination of the endoderm as in the reduced sexual zooids 

 known as sporosacs or gonophores. It may be added that if a Hydra which is 

 budding actively be starved, the buds dwindle away and sexual organs are evolved. 



The ectoderm consists of (i) covering cells, the surface of which is exposed, 

 the body and base irregular, the latter reaching the supporting lamina ; (2) epithelial 

 muscle cells, the surface of which is exposed, the base forming a muscle filament, 

 disposed in a longitudinal direction and applied to the supporting lamina ; (3) inter- 

 stitial cells, small, irregular, in masses between the bases of (i) ; (4) young cnidoblasts 

 placed deeply ; (5) fully formed cnidoblasts which are superficial ; (6) gland cells, 

 restricted to the pedal disc; (7) ganglion cells with numerous outrunners which 

 are certainly continuous, as in Eudendrium, with cnidoblasts and perhaps inter se 

 on the tentacles, as also in Eudendrium. Sense-cells appear to be wanting. A 

 ' supporting lamina ' everywhere separates ecto- from endo-derm save at the edge 

 of the mouth. It is delicate, and contains fibrils emitted from both the ecto- and 

 endo-dermal muscle cells (Jickeli). The endoderm cells are ciliated. They are of 

 three kinds (i) endodermal cells so called, which contain chlorophyl corpuscles 

 (see on Symbiosis, p. 243), are vacuolated to a certain extent and throw out pseudo- 

 podia during digestion (Parker), and also develope in the walls of the gastric cavity 

 muscle-filaments, which appear to run both circularly and longitudinally ; (2) small 

 granular gland cells in the hypostome ; (3) vacuolated gland cells at the base 

 of the gastric cavity. According to Jickeli cnidoblasts are present in the endoderm 

 both of Hydra and Eudendrium. The chlorophyl corpuscles are present chiefly 

 in the marginal part of the endoderm cells, only when very plentiful in their bases. 

 They are spherical, and consist of an outer envelope containing chlorophyl, usually 

 entire but sometimes in plates, and central protoplasmic contents. They therefore 

 closely resemble the chlorophyl bodies of plants. 



It was formerly asserted that a Hydra could be turned inside out, and 

 continue to live in this condition (Trembley). More modern researches have dis- 

 proved the statement. But it appears that within certain limits the animal can 

 be propagated^ by artificial division. 



Hydra. Kleinenberg, Leipzig, 1872 ; Jickeli, M. J. viii. 1882 \ Figures, Atlas of 

 Practical Elementary Biology, Howes, 1885. 



Tentacle. Development, Jung, M. J. viii. 1882. Pedal disc. Hamann, J. Z. 

 xv. 1882, p. 552. 



Muscle cells. Korotneff, A. Z. Expt. v. 1876. 



Endodermal pseudopodia. T. J. Parker, P. R. S. xxx. 1880. Chlorophyl cor- 

 puscles. Ray Lankester, Q. J. M. xxii. 1882 ; Id. Nature, xxvii. 1882-83 j Hamann 

 Z. W. Z. xxxvii. 1882 ; Id. Z. A. vi. 1883 ; Brandt, Z. A. vi. 1883. 



Sexual organs, Marshall, Studies Biol. Lab. Owen's College, i. 1886. 



