the Reproductive System in the Corynidae and Sertulariadse. 3 



The typical gonophore presents an external investment (ecto- 

 theca) ; a second investment (mesotheca), which lies immediately 

 within the eetotheca; 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 

 ccenosarc ("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 eetotheca 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 eetotheca, believing it " to be better to avoid all chance of 

 confounding the detached reproductive organ of a hydrozoon 

 with a truly independent organism f 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 

 Corynidse and Sertulariadse, where the eetotheca is invariably 

 present, it is not properly the gonophores that are medusiform, 

 but rather that part of them which is contained within the eeto- 

 theca. In the Calycophoridse and Physophoridse (?) the eetotheca 

 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 w 

 — 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 medusae (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 differentia involved 

 in the above definitions are sufficiently decided for all purposes 

 of description. 



1* 



