396 REPORT—1863. 
an additional membranous sac or mesotheca, with gastrovascular canals, be- . 
tween the endotheca and ectotheca (Tubularia indivisa) (fig.6 C). Ihave 
never succeeded in following the development of the mesotheca, and cannot 
say under what condition it begins, or how it proceeds, the membrane appear- 
ing always fully formed from the moment it is recognizable. It is, however, 
by no means improbable that it grows up as a cup from below, beginning 
along the line where the endotheca and ectotheca are continuous with one 
another, 
It will be seen that in the above account I differ in some important points 
from the interpretation given by Agassiz to the appearances which present 
themselves in the development of the adelocodonic gonophore. In his account 
of this process in Clava leptostyla, Agass., Agassiz* regards the walls of the 
gonophore as simple, and as homologous with the umbrella of a medusa. In 
Clava multicornis, however, the existence of two membranes may with care 
be demonstrated in these walls, though I admit that I have frequently failed 
in detecting more than a single one. In no case, however, can the walls of 
the gonophore in Clava be regarded as the homologue of an umbrella. When 
two membranes can be demonstrated in them, these will be an endotheca and 
ectotheca; if only a single membrane be present, as Agassiz believes to be 
the case in his Clava leptostyla, this will be an endotheca, while the part 
which would really represent an umbrella, namely a mesotheca, is not deve- 
lopedt. 
oh aii in the gonophores of Hydractinia polyclina, Agass,, Parypha crocea, 
Agass., and Thamnoenidia spectabilis, Agass., Agassiz correctly figures the two 
membranes which enter into their walls; but he assuredly assigns an in- 
correct origin to the more internal of these membranes when he describes it 
as rising, subsequently to the formation of the generative mass, from the 
proximal end of the gonophore in the manner of a cup closely pressed against 
the outer wall, and, at least in Hydractinia and Thamnoenidia, ultimately 
closing over the contained structures so as to form a continuous internal wall. 
Now the internal wall in the gonophore of Hydractinia is undoubtedly 
formed, not after, but simultaneously with the appearance of the generative 
mass, and is nothing more than the internal of the two layers into which 
the ectoderm of the primary bud has become divided simultaneously with its 
separation from the endoderm by the interposition of the generative elements ; 
it is thus the endotheca of the sporosac, while the more external layer is the 
ectotheca. 
Having had no opportunity of examining the development of the gono- 
phore in Parypha or Thamnocnidia, I am unable to bring any direct obser- 
vations into opposition with the views of Agassiz as to the gonophores of 
these genera; but the analogy of Hydractinia and of other hydroids, whose 
adult gonophores correspond in all essential points with those of the American 
forms, leads me to believe that the process is in all the same as in Hydractinia. 
It is only in those cases where a mesotheca becomes developed, as in Tubu- 
laria indivisa, that the adelocodonice gonophore presents any true representa- 
tive of the umbrella of a medusa, the mesotheca being properly the homo- 
* Op. cit. vol. iv. p. 221. 
+ In my earlier researches into the anatomy of the reproductive system in the Hydroida 
(“On the Anatomy and Physiology of Cordylophora,” Phil. Trans. 1853), I entertained 
the view here maintained by Agassiz, as to the homology of the parts in question. Sub- 
sequent more extended observations, however, have induced me to modify in some respects 
the views then expressed, and to adopt those which are advocated in the present Report. 
(See my paper “On the Reproductive Organs of Sertularia tamarisea,” in the Report of 
the British Association for the Adyancement of Science, 1858.) 
