180 On a Mode of Nutrition among the Hydromedusae. 
absolutely necessary that the Medusa should be nourished in 
some way or another to be able to grow; it remains there¬ 
fore to discover the means by which the nutrition of the nor¬ 
mal animals is replaced in the case of the anomalies in 
question. After searching through all the possible means, I 
can only rest upon a hypothesis which seems to me the only 
probable one. We are led by the facts to admit that the 
Medusa can nourish itself by means of its ectoderm by absorb¬ 
ing the organic material dissolved in the sea-water. 
This supposition is the more probable since, as I have de¬ 
monstrated in the case of the sponges *, these, in certain 
cases, also nourish themselves upon organic matter dissolved 
in sea-water, and also by means of their ectoderm. The com¬ 
paratively large number of anomalies of this kind that I have 
been able to observe prove that it is by no means an impossible 
thing, or even very difficult, for a Medusa to dispense with 
its entoderm in order to live and attain nearly its normal size, 
just as is the case in certain sponges. We must therefore 
conclude that, in certain cases at leastf, the ectoderm may 
fulfil the function of the entoderm ; that is to say, it may, 
as well ^as the latter, extract and assimilate the organic 
matter dissolved in water J. That we have really to do with 
organic matters dissolved in the water, and that it is not in 
the form of solid particles that the nourishment is absorbed, 
cannot be doubted; for the examination of the surface of the 
Medusa proves that it never contains any such particles. We 
should thus have two cases of such nutrition almost completely 
proved—in the class of Sponges, and in that of the Hydrome- 
duste. What has just been stated may be summarized as 
follows :— 
1 . Two species of Medusae (of the genus Bougainvillea) 
present pretty commonly an anomaly which consists in the 
complete absence of the manubrium and buccal orifice, and 
which thus presents no communication of the gastrovascular 
system with the circumambient water. 
* C. Mereschkowsky, “ Etudes sur les fiponges de la Mer Blanche,” 
Mt5m. Acad. Sci. St. Petersb. tome xxvi. no. 7 (1878), pp. 12,18. 
f I say “ at least,” because it is more than probable that in the nor¬ 
mal condition the ectoderm also assists in nutrition. There is no reason 
to endow the ectoderm with this faculty in one case, and to deprive it 
of it in another. It is probable that, in the two classes of the Sponges 
and the Hydromedusfe, the two layers are not yet entirely differentiated, 
although the experiments of Trembley with Hydra and the conversion of 
its entoderm into ectoderm have not been confirmed by the recent experi¬ 
ments of M. Engelmann (see 1 Zoologischer Anzeiger,’ 1878, no. 4). 
( It is probably for this reason that we so often meet with this anomaly, 
that the Medusa can do without the entoderm, and yet nourish itself and 
grow. 
