EEPOET ON THE HYDEOIDA. 
83 
Gonosome . — Gonangia simple capsules destitute of marsupial chamber. 
The character here assigned to the family of the Idiidse includes certain points of 
structure which must be regarded as of high systematic value, rendering necessary the 
allocation of the family to a separate section or legion of the Calyptoblastea. To this 
section the name of Thalamophora may be assigned (see Introduction, Scheme of Hydroid 
Classification). 
The facts which appear to justify this view will be obvious from the anatomical details 
given below under the description of the genus. 
Idia, Lamouroux. 
Iclia, Lamouroux, Polyp. Coral, flex., p. 199. 
Generic Character. Trophosome . — Hyclrocaulus consisting of a main stem with 
pinnately disposed rarnuli which support numerous alternate hydrothecse adnate to one 
another along the mesial line of the ramulus. 
Gonosome . — Gonangia (in the only known species) urn-shaped capsules springing from 
the main stem. 
The characters given above are all that are needed for the generic diagnosis of Idia, 
and will prevent its being confounded with any other genus hitherto described. From 
Desmoscyphus, whose hydrothecse are, as in Idia , connate to one another along the mesial 
line of the ramulus, it may at first sight appear not easily distinguishable. But we must 
bear in mind that while in Desmoscyphus the hydrothecse are opposite, they are in Idia 
strictly alternate, though notwithstanding this alternate disposition, the close approxima- 
tion in a longitudinal line of the hydrothecse composing each linear series allows of the 
two series laterally coalescing with one another in the median line of the pinna. 
It is, however, when we come to examine its anatomical structure that we meet with 
features in Idia so peculiar and so distinct from those of any other known Hydroid, that 
we are compelled to regard this remarkable form as the representative of an entirely new 
The specimens of Idm pristis obtained by the Challenger have the soft parts of the 
trophosome fairly well preserved, and though the oral extremities of the hydranths with 
the tentacles have entirely disappeared, the coenosarc is sufficiently perfect to allow of a 
satisfactory determination of some of its more important features. 
Perisarc . — The skeleton or chitinous perisarc of Idia yiristis is very remarkable. In 
every ramulus or pinna we must distinguish two aspects : — A front aspect along which 
the hydrothecse are adnate to one another by their opposed sides, and a posterior aspect 
on which they do not show themselves. 
On the anterior side (PI. XXXIX. fig. 2) the union of the adnate hydrotheca- walls may 
