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1885.] Embryology. 1236 
length of the animal, the endodermic cells are large and vacuola- 
ted as in Hydra, but the excentrically placed nuclei are absolutely 
and relatively very much smaller than in Hydra. There is a 
gastric cavity extending down from the mouth for about the same 
distance, below which the alimentary cavity becomes less apparent 
and is surrounded by solid endodermic cells. These details are 
shown in both the longitudinal and transverse sections. In the 
cross-sections evidences of slight irregular foldings of the endo- 
derm are present as in Hydra. 
Greef has described a homogeneous cuticula covering the basal 
part of the ectoderm of Protohydra. A similar homogeneous 
cuticula seems to loosely invest the ectoderm in the sections of 
Microhydra, Mr. Potts, however, is not disposed to agree with 
me on this point, as he informs me by letter that he was unable to 
detect any such envelope in the living larvz studied by him. 
There is a distinct but small oral opening at the upper end of 
Microhydra, which is cleft-like or irregular, through which food 
is taken in, as observed by Mr. Potts. The gastric cavity is small 
and only the upper end of it appears to possess a specially devel- 
oped digestive function. The digestive cavity of Microhydra is, 
in fact, but little more developed than in the later planula-stage 
of such a form as Eucope. In fact, if the planula of the latter 
were to lose its cilia, become fixed and acquire a mouth, the 
morphological complexity of Microhydra would be realized. We 
have then in Microhydra an adult type which represents prac- 
tically a planula which has acquired a mouth. In other words, 
the new type is not only the simplest of Hydroids, but is also the 
simplest of all true polypes or Coelenterata. It is, in fact, a much 
more rudimentary form than even Protohydra, and represents 
perfectly a permanent gastrula which reproduces itself by lateral 
gemmation instead of by transverse fission. 
It has been contended by some that Hydra is a degenerate 
form, and that Protohydra and Microhydra must be considered in 
the same light. As no very cogent reasons have ever been ad- 
duced in support of such a conclusion, I shall leave the Onus 
probandi to be produced in favor of that view by those with whom 
it originated. The very simplicity of the type in question, it seems 
to me, must ever remain a serious bar to arriving at any very cer- 
tain conclusions on this head. It seems as impossible to me to 
prove that Microhydra has been developed by the degeneration 
of a higher type, as it does to prove that it is a form which has 
advanced but very little beyond the planula stage of the Ccelen- 
terata. The latter view seems to me to be by far the most prob- 
able, since it is a free-living form, which is no more likely to have 
been adversely affected than would hundreds of others living in 
the same environment. The significance of this singularly inter- 
esting type in relation to the question of the possible origin of 
the Ccelenterata is very great, and the interest which attaches to 
