REPOET ON THE HYDROLDA. 
65 
Gonosome . — Gonophores adelocodonic ; gonangia springing from one side of the 
hydrocaulus, no marsnpium present in either sex. 
The genus Thuiarici was separated by Fleming from Sertularia on grounds derived 
from the supposed immersion of the hydrothecse in the hydrocaulus, instead of their 
being as in Sertularia merely adnate to its surface. 
The character, however, thus relied on has really no existence. The hydrothecse of 
the species separated on this ground from Sertularia are not more deeply immersed in 
the hydrocaulus than are the hydrothecse of most species of Sertularia, and the 
appearance of immersion is given by the greater extent in which the walls of the 
hydrothecse are in some species adnate to the hydrocaulus, while even this is never 
greater than what occurs in many species wdiich are allowed to remain in Sertularia. 
There can, however, be no doubt that the general aspect of Thuiaria contrasts 
strongly with that of the species with which it had been previously associated, but the 
true source of this is to be found in a character very different from that of the supposed 
immersion of the hydrothecse in the hydrocaulus. 
In the species of Sertularia with opposite hydrothecse every pair of hydrothecse has 
an internode of the stem to itself, being separated on each side from the adjacent pair by 
a joint. Iu those species in which the hydrothecse are alternate the internodes carry 
each, sometimes two alternate hydrothecse, but most frequently a single one. In these 
various cases the intervals between the joints are constant, every internode in the same 
colony carrying the same number of hydrotheca. 
In Thuiaria it is different. The internodes here always carry many hydrothecse, 
whether in pairs or alternate. The joints moreover are in most cases so decided as to 
form well-marked constrictions in the stem, a character which in combination with the 
length of the internodes confers upon Thuiaria an aspect very striking and distinctive. 
Moreover, the internodes are often of variable length, the number of hydrothecse carried 
by each frequently varying in different parts of the same colony. 
It is true that in Sertularia we may sometimes meet with examples in which for a con- 
siderable length of the stem no joints are apparent. This, however, is a secondary con- 
dition resulting from the effacement in the older parts of the hydrocaulus of the joints which 
had been present in an earlier period ; 1 while in Thuiaria the very well marked joints 
are at all times separated from one another by intervening series of many hydrothecse. 
If the diagnostic characters of Thuiaria be such as are here contended for, some 
species which had been hitherto referred to Sertularia must be removed to Thuiaria. 
Sertularia cupressina, Linn., and Sertularia argentea, Ellis and Solander, will thus 
find their places in Thuiaria. Indeed there is not a single character which would justify 
the inclusion of these Hydroids in Sertularia rather than in Thuiaria. 
1 I believe that the Thuiaria sertularoides of The Hydroids of the Gulf Stream is a case of this kind, and that the 
species is truly a Sertularia which, on account of the effacement of the joints, I have erroneously referred to Thuiaria. 
(zool. chall. exp. — part lxx. — 1888.) Aaaa 9 
