84 
THE VOYAGE OF H.M.S. CHALLENGER. 
be seen to form on the surface of the ramulus a continuous zigzag line lying in the general 
direction of its axis. The posterior side, on the contrary, is destitute of markings of 
any kind, but when focused at a little distance below the surface (fig. 3) there will be 
brought into view two longitudinal series of irregularly polygonal areolae, those of each 
series alternating with those of the other, and being throughout in close contact with 
them. They are bounded by chitinous laminae which pass off from the anterior walls of 
the common perisarcal tube, and penetrating into the interior of this, give rise to two 
longitudinal series of chambers which are lined by the coenosarc in the way about to 
be seen. 
Between the posterior walls of the ramulus and this double series of chambers is a 
space occupying the entire width of the ramulus, and forming a posterior longitudinal 
chamber which extends continuously through the entire ramulus (fig. 5 ; fig. 8, c). 
The hydrothecae terminate each in a very oblique orifice, which is directed posteriorly 
and is provided with a delicate membranous valve (figs. 3, 4, 9). 
Coenosarc , — All the areolae or anterior chambers just mentioned are lined by the 
coenosarc, which instead of forming as in other Hydroids a continuous uninterrupted tube, 
is broken up into two longitudinal series of segments (figs. 4, 5 ; fig. 8, b). There can 
be little doubt that the segments of one and the same series freely communicate with 
one another through the chitinous walls of the chambers. 
The coenosarc by which these segmental chambers are lined consists of a thick 
endodermal layer overlaid by a thin ectoderm, and enclosing that portion of the common 
somatic cavity which lies in each segment. With each of these segments a hydranth 
is directly continuous (fig. 5), and every hydranth thus corresponds to one of the 
transverse segments into which the common coenosarc of the ramulus is divided, and has 
its gastral cavity opening directly into the cavity of the segment. 
The great longitudinal posterior chamber (fig. 5 ; fig. 8, c) is also lined by a layer of 
coenosarc. This, however, is thinner than that of the anterior transverse chambers, and was 
but imperfectly preserved in the specimen. 
The pinnately disposed ramuli of Idia pristis thus present a structure which, so 
far as is known, does not occur elsewhere among the Hydroida. The division of the 
common chitinous perisarcal tube into distinct chambers, and the consequent division 
and segmentation of the coenosarc, constitute a combination of characters which, how- 
ever far we may be from assigning to it its true significance, is of sufficient 
systematic value to necessitate the relegation of Idia to a separate section among the 
Calyptoblastic Hydroids. 
While the structure of the pinnae is thus so very exceptional, that of the common 
stem (fig. 6) differs but little from the usual condition of this part in the Sertularian 
Hydroids. The hydrothecae are as in the pinnae alternate, but those of opposite sides 
show no tendency to coalesce, being on the contrary separated from one another by a 
