REPORT ON THE HYDROIDA. 
57 
compared with that of the various species of Halecium. While in these, however, 
the hydrothecae are evanescent, and not only the tentacular crown but the body of the 
hyclranth always remains uncovered, in Sertularia exsertci and in Thuiaria hyalina the 
hydrothecae are especially well developed, and though the tentacular crown in these two 
Hydroids always remains exposed, the body of the hydranth remains under cover of the 
hydrotheca. 
Another feature of great interest and significance is found in the fact that in the present 
species every tentacle is provided at its base wfith a remarkable organ in the form of a little 
cushion-like prominence loaded with slightly curved, rod-shaped thread-cells (figs. 15, lc). 
It is scarcely possible not to recognise in these little batteries of thread-cells defensive 
organs compensating for the loss of the protection which in other cases is afforded by the 
hydrothecse. Whether, however, similar organs occur in the equally exposed liydranths 
of Thuiaria hyalina the state of the specimen did not allow me to determine ; while we 
cannot overlook the fact that in the still more exposed liydranths of Halecium no such 
organs have been detected. 
The gonangia of Sertularia exserta are exceedingly beautiful. The thin but very 
prominent annular ridges with which they are encircled give to them the appearance of 
a symmetrical pile of discs, while the profusion in which the gonangia are developed gives 
to the entire colony an aspect no less pleasing than striking. 
The specimens in the collection have a height of between one and two inches, while 
the comparatively good state of preservation in which the soft parts are retained allowed 
of a satisfactory determination of the points here described. 
Sertularia echinocarpa, n. sp. (PI. XXVIII. figs. 1, la). 
Trophosome. — Stem fascicled towards the root, becoming monosiphonic distally, 
pinnately branched, branches alternate. Hydrothecse alternate, large, tubular, cylindri- 
cal, slightly tumid towards the base on the apocauline side, free for about two-thirds 
of their height, orifice quite even and circular. 
Gonosome. — Gonangia springing each by a short peduncle from a point close to the 
base of a hydrotheca, pyriform, thickly set with hollow, blunt, spine-like outgrowths of 
their chitinous perisarc. 
Locality.^- Station 149 d, Royal Sound, Kerguelen Island; depth, 28 to 60 fathoms. 
Sertularia echinocarpta is a large and strong form, attaining a height of upwards of six 
inches. The main stem is slightly wavy and set with hydrothecse along its entire length. 
It sends off at nearly equal intervals shorter pinnately disposed alternate branches, which 
are set with hydrothecse and differ in no respect except in length from the main stem. 
The long tubular hydrothecse, free for the greater part of their height, afford a very 
(zool. chall. exp. — pabt lxx. — 1888.) Aaaa 8 
