326 - M. C. Mereschkowsky on the Hydroida. , 
slender hydranths, all unmistakably recall a Hydractinia or a 
Podocoryne ; so that from the trophosome alone it is impossible 
to distinguish Oorhiza from one of the above-mentioned genera. 
But the sexual individuals (gonosomes) present such charac- 
teristic peculiarities that it is impossible to place the Hydroid 
in question in any of the known genera of Hydractinide. 
Podocoryne, as is well known, has the gonosomes consisting of 
blastostyles upon which Medusz are produced ; Hydractinia 
differs in that the same blastostyles give origin to sporosacs ; 
lastly, Oorhiza has no blastostyles at all, but the sporosacs 
rise from the hydrorhiza itself. 
It is true that in Podocoryne the presence of the blasto- 
styles is not a constant character: their absence, indeed, is 
constant in Podocoryne areolata; in others, as for example 
Podocoryne aculeata, there may or may not be a blasto- 
style; and, finally, in Podocoryne carnea the Meduse are 
constantly produced upon blastostyles. But this does not 
apply to the genus Hydractinia, nm which the presence of 
blastostyles is a perfectly constant character and essential to 
the genus. 
The genealogical relations between the genera and species 
of the family Hydractiniide may be expressed by the accom- 
panying diagram :— 
echinata. polyclina. 
Hydractinia  Oorhiza borealis. P. areolata. P. carnea. 
\ / 
s Z 
S f 
Intermediate form*. ; Podocoryne aculeata. 
SS : 
Bg 
? 
* This hypothetical intermediate form must have existed analogically 
with Podocoryne aculeata ; it must have possessed indifferently sporosacs 
on blastostyles and without blastostyles: on one side Hydractinia was 
developed (analogically with Podocoryne carnea); on the other Oorhiza 
(analogically with P. areolata). If ever this hypothetical form is 
found, it will be necessary to unite the genera Hydractinia and Oorhiza 
into a single one, or else to establish a distinct genus for Podocoryne 
aculeata, which can by no means be done, 
