86 
THE VOYAGE OF H.M.S. CHALLENGER. 
entire course short pinnae which differ hut little in length. The wide angle at which the 
distal ends of the hydrothecae diverge from the pinnae, and the narrow point in which 
they terminate, give to the pinnae with its two rows of hydrothecae a not very distant 
resemblance to the saw of a sawfish, — a resemblance which suggested to Lamouroux the 
specific name of pristis. 
The pinnae spring by a narrow base, each from a short cladophore. Some of them 
present one or two well-marked constrictions, which are not situated at any regular 
distance from one another, and by which the pinna becomes divided into two or more 
internodes of variable length. Some of the pinnae, however, show no constriction in any 
part of their course, and no trace of division into distinct internodes. 
The distal extremities of the hydrothecae in both pinnae and main stem, after 
diverging at a wide angle end in a point which forms the apocauline boundary of a wide 
orifice. The plane of this orifice in the pinnae is directed backwards, while in the stem 
it looks, more towards the distal end of the colony. In both the orifice is completely 
covered by a thin membrane, the distal half of which is free and capable of being raised 
from the orifice in the form of a valve-like lid. 
The structure of the stem differs widely from that of the pinnae, and in most respects 
resembles the usual structure of this part in the Sertularian Hydroids. It is divided by 
well-marked joints into a series of equal internodes, each of which sends off a pinna from 
alternate sides near its proximal end. The hydrothecae, which are deep and nearly 
cylindrical, are alternately disposed and are confined to one aspect of the stem, which 
thus presents as in the pinnae an anterior and a posterior aspect, but the hydrothecae of 
the stem show no tendency as in the pinnae to coalesce with one another along the mesial 
line. 
The hydro thecae of the stem do not open directly into the common tube, but each 
communicates with it through the medium of a small chamber, which forms an appendage 
to the base of the hydrothecae and opens by a well-defined circular orifice into the cavity 
of the stem. These little chambers are probably the representatives of the much larger 
adnate chambers into which the hydrothecae open in the pinnae. 
The gonangia arise each from a point close to the base of a hydrotheca, and care 
must be taken not to mistake the optical expression of the orifice of the accessory chambers 
just mentioned for the spot from which a gonangium had become detached. The. short 
neck, which carries on its summit the orifice of the gonangium, is encircled by a wreath 
of minute puncta, which recalls a very similar condition in the limbus of the hydrophore of 
Halecium. 
A very interesting feature in the economy of Idia pristis consists in the apparent 
ramification of the stem. In all the specimens which I have examined, whether those 
brought home by the Challenger or those in the possession of Mr. Busk, the main stems, 
besides sending off the pinnae, have the appearance of dividing into a greater or smaller 
