HISTOLOGY OF HYDRA FUSOA. 
223 
hellaria and in sponges.^ The Eussian 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 Kydra adds another instance 
to the two already brought forward by Metschinkoff, of a Meta- 
zoon exhibiting what is usually considered to be a distinctively 
Protozoan mode of digestion. It is quite possible that a pre- 
liminary disintegration of the animals taken in is performed by 
juices secreted by the endoderm ceils, 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 end of the body in possessing large vacuoles. Their 
nuclei are in some instances, although not constantly, simple 
and non-nucleolate like those of the ectoderm cells from the 
same region. 
Finally, I have been able fully to confirm Professor Huxley’s 
statement^ as to the presence of nematocysts in the endoderm 
(fig. I, 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 nematocysts. 
4. Methods . — For sections, the Hydros were either killed with 
hot water, and placed in Kleinenberg’s picric acid for two hours, 
or were placed alive in ammonic bichromate, I 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 speci- 
mens 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 (O’ 5 
’ ‘ Zool. Anzeiger,’ Bd. I, (1878), p. 387, and ‘ Zeltsch. 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 Met.s* 
chinkoff’s. 
2 Huxley and Martin, ‘ Elementary Biology,’ p. 100, 
