Se Ta oe ee rh nl al a 
the Reproductive System in the Corynide and Sertulariade. 3 
The typical gonophore presents an external investment (ecfo- 
theca) ; a second investment (mesotheca), which lies immediately 
within the ectotheca; a third one (endotheca), situated imme- 
diately within the mesotheca ; a central, more or less elongated 
body (spadix), which lies in the axis of the gonophore, and con- 
tains a cavity in free communication with that of the polype or 
coenosare (“somatic cavity” of Huxley); and lastly, the genera- 
tive elements (ova or spermatozoa), which surround the spadix 
and are themselves immediately invested by the endotheca. 
The ectotheca is a simple extension of the ectoderm of that 
part of the zoophyte from which the gonophore arises, and it 
encloses either a sporosac or a medusoid. 
Professor Huxley objects to the use of the term “ medusa,” 
by which I have hitherto designated one form of the contents of 
the ectotheca, believing it “to be better to avoid all chance of 
confounding the detached reproductive organ pf a hydrozoon 
with a truly independent organism ;” and he prefers the expres- 
sion “ medusiform gonophores.” 
There is value in this criticism, and I by no means desire to 
insist on the retention of the word “medusa;” but “ medusi- 
form gonophore” does not express my meaning; for in the 
Corynide and Sertulariadz, where the ectotheca is invariably 
present, it is not properly the gonophores that are medusiform, 
but rather that part of them which is contained within the ecto- 
theca. In the Calycophoride and Physophoride (?) the ectotheca 
is apparently obsolete, at least after the very early stages of the 
gonophore have been passed, and the expression “ medusiform 
gonophore ” would then be quite applicable ; all difficulty, how- 
ever, will be avoided by the adoption of the term “ medusoid ” 
—a term which has already been for some time in use. 
The mesotheca, endotheca, and spadix of the gonophore may 
all enter into the composition of a sporosac (e. g. Tubularia 
indivisa) ; they or their homologues must all enter into that of a 
medusoid. 
When the mesotheca is present and contractile, the body is a 
medusoid ; when it is absent or non-contractile, the body is a 
sporosac. The mesotheca becomes in the medusoid an umbrella 
( gonocalyx ” of Huxley). 
Professor Huxley maintains that the transition between what 
I term sporosacs and medusz (medusoids) is so gradual that no 
line of demarcation can be drawn between the two, though he 
would admit the applicability of the term sporosac to such forms 
as we meet with in Hydractinia. I believe, however, that the 
distinction is a practical one, and that the differentie involved 
in the above definitions are sufficiently decided for all purposes 
of description. 
1* 
