~ ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 411 
sented by the great majority of the Hyprorpa, and described above as the 
Planula of Sir J. G. Dalyell; the other confined to the genus Tubularia, 
unless Hydra also should afford an example of it, and which may be in the 
same way designated by the term Actinula*. Every hydroid, if we except 
such forms as may be proved to pass to the medusal condition directly from 
the egg, thus commences its free existence either as a planula or an actinula. 
Import of the Phanerocodonic Gonophore.—The developmental phenomena 
now described have been all observed in ova which have their origin in an 
adelocodonic gonophore; and they enable us to trace back the hydroid in an 
unbroken series, through the egg from which it is developed, and the gono- 
phore in which this egg originates, to the hydroid trophosome from which the 
gonophore buds. 
The cases in which a similarly unbroken chain can be traced back through 
the free phanerocodonic gonophore are naturally far less frequent ; for in the 
majority of cases the free gonophore does not produce its generative elements 
until a considerable time after it has become free, and undergone more or 
less change of form as it continues to develope itself in the open sea; and it 
is very seldom that we can succeed in rearing the free meduse in the con- 
finement of our tanks up to the period when they shall attain sexual ma- 
turityt. We thus then almost always lose absolute evidence of identity in 
the gonophore when presented at two distant periods of its life; and there 
is, therefore, necessarily an interruption in the series of direct observations. 
Some such direct observations, however, have been made; and, besides these, 
so many facts have been ascertained by more or less discontinuous and frag- 
mentary observations, that there remains no longer any doubt that the 
history of the phanerocodonic gonophores and their progeny is in all essential 
points identical with that of the adelocodonic forms. 
Some cases have been observed in which the phanerocodonic gonophore 
has attained to complete maturity, and become loaded with ova or sperma- 
tozoa, before separating itself from the trophosome. 
The first recorded instance of this phenomenon seems to be that described 
by Cavolinit, who observed the medusiform zooids of his Sertularia pennaria 
(Pennaria distycha, Goldfuss) to become loaded with ova while still attached 
to the trophosome—an observation which about three-quarters of a century 
later was confirmed by M*Crady§, who saw the ova in his Pennaria tiarella 
even become developed into planule before the detachment of the gonophore. 
Rud. Wagner || saw in a hydroid, which he names Coryne aculeata, the 
formation of buds, which became developed into somewhat arrested medusa- 
like gonophores ; and then, before detaching themselves from the trophosome, 
gave origin to a copious brood of eggs in the walls of the manubrium. 
Lovén{ saw in a Coryne, which he refers to the Syncoryne ramosa of 
Ehrenberg, a medusoid body, also somewhat arrested in its development. It 
* While the present Report was in the hands of the printer, I received a letter from 
Mr. Alder, in which he informs me that he has bred from the Myriothela arctica of 
Sars, free polypoid embryos closely resembling the free stage of Zubularia. From this 
important observation, it would follow that Myriothela arctica must also be placed among 
the hydroids which commence their free existence under the form of actinule. 
t In the Srrnonoruora the opposite condition is prevalent; for here the gonophores, 
even such as present the more complete medusal or phanerocodonic form, usually become 
loaded with ova or spermatozoa before they detach themselves from the trophosome. 
{ Mem. Polypi Marini, 1785. 
§ Proc. Elliott Soc. Nat. Hist. 1857. 
| Isis, 1835, p. 256. tab. 11. 
4] Miiller’s Archiv, 1857, p. 321. 
