1879.] 



On the Histology of Hydra fusca. 



65 



will have been shown to exhibit a process of alimentation identical 

 with that described by Metschinkoff, in the lower Turbellaria and in 

 sponges.* The Russian observer describes the complete obliteration; 

 during digestion, of the digestive cavity in the Turbellarians, and of 

 the canals in the sponges ; and, in the former as well as the latter, he 

 has undoubted evidence of the actual ingestion of solid particles by 

 the endoderm cells. 



It would seem, therefore, that Hydra adds another instance to the 

 two already brought forward by Metschnikoff, of a Metazoon exhibit- 

 ing what is usually considered to be a distinctively Protozoan mode of 

 digestion. It is quite possible that a preliminary disintegration of the 

 animals taken in is performed by juices secreted by the endoderm cells, 

 but the final digestion seems to take place in the actual protoplasm of 

 the cells, into which the food articles are taken in the solid form. 



The endoderm cells of the tentacles resemble those of the proximal 

 and of the body in possessing larger vacuoles (fig. 5). Their nuclei 

 are in some instances, although not constantly, simple and non-nucleo- 

 late like those of the ectoderm cells for the same region (fig. 5). 



Finally, I have been able fully to confirm Professor Huxley's state- 

 mentf as to the presence of nematocysts in the endoderm (fig. 1, n), 

 a statement which, as far as I am aware, has not been made, with 

 regard to Hydra, by any other writer on the subject. This fact is, like 

 the absence of interstitial tissue in the tentacles, an argument against 

 Kleinenberg's view that the tissue is the sole source of the nema- 

 tocysts. 



4. Methods. — For sections, the Hydrce were either killed with hot 

 water, and placed in Kleinenberg's picric acid for two hours, or were 

 placed alive in ammonic bichromate, 1 per cent. — which always kills 

 them in the half extended condition — and kept in it for two or three 

 days. In either case they were afterwards transferred to 50 per cent, 

 alcohol, and then placed successively in 75 per cent., 90 per cent., and 

 absolute alcohol. The specimens were stained either with carmine or 

 picrocarmine, and imbedded in cacao butter, after soaking for a short 

 time first in oil of cloves and then in melted cacao butter. By this 

 means they became so thoroughly permeated with the imbedding 

 material that they could, be cut without the loss of a single section ; 

 even longitudinal sections of the tentacles could be made with ease. 



For teasing I employed ammonic bichromate, acetic acid (0*5 per 

 cent.), or osmic acid (1 per cent.), and for this purpose the specimens 

 were not transferred to alcohol, but to weak glycerine (equal parts of 

 glycerine and water ) , in which they were teased out. 



* "Zool. Anzeiger," Bd. I (1878), p. 387, and " Zeitsch. f. wiss. Zool." 

 Bd. xxxii (1879). It need hardly be said that the above view of the physiology of 

 digestion in Hydra was suggested by these papers of Metschinkoff's. 



f Huxley and Martin, " Elementary Biology," p. 100. 



VOL. XXX. F 



