278 Proceedings of the Boyal Physical Society, 
Generative sacs attached to reproductive polyp without \ Eudendrium 
mouth or tentacles ; summit of polyp surmounted by a > confertum 
cluster of large thread-cells, as in , . . . ) (Alder). 
Generative sacs or medusoids attached to reproductive ") Cionistes, 
polyp without mouth, tentacles, or cluster of thread- > Sertularia, 
cells, as in . . . . . . . . ) Campanularia. 
Reproductive polyp divided longitudinally into several ^ Eudeiidrium 
portions, each surmounted by its cluster of large thread- I arhusculum 
cells ; sperm-sacs formed, as in Hydra, by simple dila- > (T. S. W.), 
tation of the ectoderm ; each division of polyp trans- E. cainllare ? 
formed into a " moniliform" sperm-sac, as in . .J (Alder). 
fAtrax:,tylis 
(T.S.W.), 
^Mder and 
T. S. W.), 
Cordyloplwra 
(Allman). 
It will thus be seen that there is a very gradual transition 
from the alimentary polyp to the reproductive polyp, and 
from the latter to the simple generative sac. Professor All- 
man's term " blastostyle/' applied to the reproductive potyp, 
is apt to mislead, as it indicates that the alimentary and re- 
productive polyps are not homologous parts. Still more 
decidedly does that accomplished naturalist confuse the 
homology of these parts by applying the same term to the 
branched pedicle of the aggregated generative sacs of Tuhu- 
laria indivisa, which is merely formed of the conjoined and 
elongated pedicles of the individual sacs. 
It is impossible to construct any classification of the Hy- 
droid Zoophytes on the form or position of their generative 
sacs or medusoids, as these vary not only in different species 
of the same genus, but also in males and females ol the 
same species. Thus, in Eudendrium rameum the sperm-sacs 
are moniliform, the egg-sacs single ; the former are attached 
to the alimentary polyp, the latter to the polyp and also to 
the polypary. In Hydractinia, although the generative sacs 
generally spring from the reproductive polyps, they are also 
found attached to the polypary ; and in a most interesting 
species of this genus lately discovered by Mr Alder, medu- 
soids spring from the latter part of the zoophyte. InA trac- 
tylis rarnosa, T. S, W. (Eudendrium ramosum, Van. Ben.), the 
medusoids, the males and females of which differ in shape, 
spring from the polyps, from club-shaped bodies, and from 
