It cannot be urged as an argument against this view, that the circular 
canal of the medusa is not represented in the polypite; for the absence of a 
developed umbrella in the polypite necessarily brings with it the absence of 
this canal; and for the same reason, velum, lithocysts, and secondary tentacles 
are also absent. Neither can it be said that those cases in which the ten- 
tacles of the polypite are not arranged in a single verticil, but are repeated 
regularly or irregularly in different planes upon the body, are inconsistent 
with the homological relations here insisted on; for such cases can be 
regarded only as special modifications of the more typical plan which has 
directly suggested our comparison. ' 
Huxley, believing the difference in structure and development between the 
locomotive disc of the gymnophthalmic and that of the steganophthalmic 
medusze to be so great as to place them in different categories, would confine 
the term “umbrella” to the disc of the steganophthalmata, and would desig- 
nate that of the gymnophthalmata by the terms ‘“‘nectocalyx” and “gonocalyx.” 
I was at first disposed to adopt the same view; but an investigation of the 
mode in which this part makes its appearance in the gymnophthalmic forms 
has convinced me that the development is essentially the same in both cases, 
and that, notwithstanding some marked structural differences, there is suffi- 
cient unity between the two to render it more convenient to speak of them 
under the same term as strictly homologous organs. In both cases they are 
formed by an outgrowth of the walls of the polypoid manubrium, and the fact 
that the steganophthalmic medusa is produced by successive transverse divi- 
sions of a “ scyphostoma,”’ while the gymnophthalmic medusa is formed as a 
lateral bud from a hydroid trophosome, is no valid argument against this 
approximation ; for every segment of the “ scyphostoma” is strictly compar- 
able to the bud of the hydroid, and developes its umbrella by an outgrowth 
from its sides in quite the same way. 
A very instructive example, which strikingly bears out the comparison I 
haye here attempted to make between the polypite and the medusa, is afforded 
by the remarkable locomotive zooid which with its ectotheca forms the gono- 
phore of Dicoryne (fig. 4, p. 358). This little zooid is essentially a free me- 
dusa, reduced to the condition of an ova-bearing or spermatozoa-bearing 
manubrium, from whose base two free tentacula are developed. Now there is 
here no umbrella; but it is evident that we have only to imagine the ectoderm 
of the manubrium projected as a disc, in the way already supposed, in the 
horizontal plane passing through the base of the two tentacles so as to in- 
clude the basal portion of these tentacles in its thickness, in order to have 
an umbrella with two radiating canals added to the manubrium. 
But development entirely coincides with anatomy in pointing to the same 
conclusion ; and it is only necessary to trace the formation of the umbrella 
and radiating canals in the budding medusa, in order to become convinced 
that their origin is essentially that here insisted on (see below, p. 397) ; while 
the interesting observations of Johannes Miiller on the development of Aigi- 
nopsis (see below, p.418), and of McCrady on that of Cunina (see below,p. 419), 
show that in these genera the umbrella grows out as a horizontal disc from 
the walls of a free polypoid manubrium, which bears a close resemblance to 
the generative zooid of Dicoryne*. 
ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 365 
* At the same time, however, we must not, in this comparison, overlook the fact that 
both Aginopsis and Cunina belong to the AHginide, a family which in many respects pre- 
sents an approach to the steyanophthalmata ; while, according to Fritz Miiller’s account 
of the development of Lyriope catharinensis (Wiegm. Arch. 1859, p. 310), the process would 
seem to be, even in the undoubted gymnophthalmata, sometimes different ; asin this case, 
