ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 361 
the mouth of the bell, where it forms a thin muscular diaphragm, perforated 
in the centre by a circular opening of greater or less diameter. 
7. Generative elements *. 
In the description here given of the phanerocodonic gonophore, the some- 
what aberrant group of meduse which constitutes the family of the Ziyinide 
is not included ; for these meduswe, while further research will probably suc- 
ceed in referring them to polypoid trophosomes, have not as yet in any case 
been actually so traced. (See below, p. 418.) 
We have already seen that the phanerocodonie gonophore may be a truly 
sexual zooid, which will then always give origin to ova or spermatozoa by the 
direct development of these elements between the ectoderm and endoderm of 
its manubrium (see fig. 5 B), without the intervention of any specialized bud, 
and will accordingly in this respect entirely correspond with the adelocodonic 
forms. It is to these properly sexual phanerocodonic gonophores that I have 
given the name of “gonocheme.” Of this form we have examples in the 
types which have been described by authors under the names of Sarsia, 
Steenstrupia, Oceania, &e. (figs. 6 D, 8, & 16). 
A large number of free gymnophthalmic meduse, however, some of which 
are known to proceed from polypoid trophosomes, are properly non-sexual, 
and cannot give origin to the generative elements without the previous deve- 
lopment of a special sexual bud. The sexual bud is borne upon some part 
of the course of the radiating canals; and it is this bud which is the true 
physiological equivalent of the adelocodonic gonophore and of the sexual form 
of phanerocodonic gonophore. Examples of this phenomenon may be seen 
in those medusze which have been included by authors under the types of 
Obelia, Eucope, Thaumantias, &e. (figs. 1g, 17, & 18). 
It is to these free medusx, which, while they are themselves properly non- 
sexual, give origin, like the gonoblastidium of Hydractinia, to sexual buds, 
but which, unlike the gonoblastidium, are endowed with locomotive powers, so 
that they carry these buds from place to place by the contractions of their 
umbrella, that I propose to give the name of “ gonoblastocheme’’+, 
In the account here given of the gonoblastocheme, I have confined this term 
to such forms of medusze as develope distinct sexual buds upon the radiating 
canals. In some of those medusx, however, in which the reproductive 
elements are produced between the ectoderm and endoderm of the manu- 
biium, we find a greater or less tendency to a differentiation of the repro- 
ductive mass from the general walls of the manubrium. This may be seen, 
for example, in certain forms of the Oceania, Turris, and Lizzia types, in 
which the ova or spermatozoa are developed in more or less specialized, fre- 
quently convoluted lobes of the manubrium, while in some, as in Nemopsis, 
Agass., these lobes are continued from the manubrium for some distance 
along the course of the radiating canals. Iam not, however, yet prepared 
to place these cases in the same category with the true gonoblastocheme ; for 
I have not had an opportunity of examining the structure of the manubrial 
* The structure above described is that of the phanerocodonic gonophore in its most com- 
pletely developed form, such as we meet with it in many of the Tubularida and Campanu- 
larida. Such complete differentiation, however, is not always attained even in the Hy- 
DROIDA, while among the StpHonoPHORA the margin of the umbrella in the Calycopho- 
ride carries neither tentacles, ocelli, nor lithocysts, and the manubrium developes, at least 
usually, no mouth upon its extremity. ‘ 
+ In order to avoid complicating the subject, I have deemed it better to leave out of view 
the question as to whether the particular meduse under discussion have been derived from 
a polypoid trophosome, or are the direct result of the development of anovum. (See below, 
p. 417 &c., where this question of the origin of the medusz is fully treated.) 
