REPORT ON THE HYDROIDA. 
85 
wide interval of the stem, into the cavity of which that of the hydrotheca opens through 
the medium of a short tubular continuation of its proximal end. 
The perisarcal tube of the stem is simply continuous and monothalamic, and its con- 
tained ccenosarc is consequently quite destitute of the peculiar segmentation which is 
so striking in the pinnse. The coenosarc of the stem lines the perisarc with a rather 
thin layer, whose endoderm is thrown into an irregular network of tubular ridges, a 
condition which prevails also in Antennularia, and is elsewhere not without its parallel 
among the Hydroida. 
Idia pristis, Lamouroux (PL XXXIX. figs. 1-10). 
Idia pristis, Lamouroux, Polyp. Coral, flex., p. 200, pi. v. fig. 5. 
Trophosome . — Stem monosiphonic, simple, 1 springing from a plexus of tubular 
filaments, and sending off along nearly its entire length moderately long, rather close-set, 
alternate pinnse, which are for the most part intersected by transverse joints at distant 
and variable intervals. Hydrothecae of pinnae alternate, with their opposed sides adnate 
to one another, and with the free distal end diverging at a wide angle from the liydro- 
caulus, tapering to a point and opening by a very oblique orifice which is directed 
backwards, and closed by a membranous valve ; hydrothecae of stem not adnate by their 
opposed sides. 
Gonosome . — Gonangia borne exclusively by the main stem, each springing from a 
point just below the base of a hydrotheca, urniform, longitudinally fluted, attached to 
the stem by a short but definite peduncle ; summit abruptly narrowed into a conical 
roof, which bears on its apex the wide, circular orifice. 2 
Locality. — Station 203, off Panay, Philippine Islands; lat. 11° 61' N., long. 123° 9' 
E.; depth, 20 fathoms. 
Off Bahia ; depth, probably 10 to 20 fathoms. 
Idia pristis is the only known representative of the genus ; and the only descrip- 
tion of it hitherto published consists in the short and inadequate diagnosis given by 
Lamouroux. 
It occurs in the form of groups of undivided stems which send off along nearly their 
1 The apparently branched condition of the main stem is due to a cause different from that ot a true ramification. 
See below, p. 86. 
2 Lamouroux, to whom we owe the original definition of the genus Idia , knew nothing of the gonosome, and, 
except in a short notice by Mr. Hincks (Journ. Linn. Soc. Lond. (Zook), vol. xxi., 1887), this important part of the colony 
has hitherto remained undescribed. Mr. Busk, however, had many years ago examined it in specimens from the 
Persian Gulf, and I am indebted to him for an opportunity of inspecting the excellent figures which he then made 
of the gonangia. He has also placed his specimens in my hands, and I have thus been enabled to compare these 
with the examples brought home by the Challenger. One of the examples of Idia ‘pristis, dredged by the Challenger 
off Bahia, is furnished with gonangia. No gonosome is present in any of the other specimens. 
