320 MR. W. B. S. BENHAM ON THE MUSCULAR 
cavity above; the sides of this bag may be termed the anterior and posterior lamelle. 
Across the middle line, for a short distance on each side, these two lamelle are free 
from one another and membranous, and are produced in the middle line ventrally as a 
membranous tongue-like appendix called the sternal lobe (Pl. LXXIII. fig. 4, m/), 
containing a space continuous with that between the lamelle. 
A single branchiferous appendage, considered apart from its fellow to which it is 
joined across the middle line, consists of a broad, flat, chitinous basal piece, which 
carries the gill-book on its posterior face. From this basal joint there springs a broad 
chitinous exite on the outer side, and on the inner side the limb continues in three 
joints, the last of which hangs pretty freely downwards at the side of the membranous 
tongue already spoken of as the sternal lobe (see Pl. LXXIIL. fig. 4). The gill-book is 
placed on the basal joint outside the posterior lamella, and consists of about 150 double 
leaves, the double leaf being a flattened bag of two plates opening into the space 
between the anterior and posterior lamelle of the appendage. Of these the smallest is 
placed anteriorly, and the largest posteriorly, each one overlying the succeeding lower 
one. 
The anterior lamella of the branchiferous limb is strengthened by two chitinous bars, 
one going obliquely outwards, the other passing downwards along a flat chitinous plate, 
which is situated just outside the sternal lobe. To these chitinous pieces some of the 
muscles of the appendage are attached. On the posterior lamellz are also one or two 
small sclerites (see Pl. LX-XIII. fig. 4). 
Close to the base of the sternal lobe, on each side, and close to the middle line, is 
situated a stigma (sty); this leads into a hollow tendon, which passes upwards and 
forwards for about 2 inch, and in its anterior end is inserted a muscle. The six muscles 
from these tendinous stigmata on each side form the two large branchio-thoracic 
muscles, which raise the floor of the abdomen by their contraction. 
The chitinous supports of the anterior lamelle have a similar structure to that of the 
other chitinous parts, but bear some very curious large hairs inserted in cups situated 
in the outer layer of chitin. These compressed hairs are of two sorts, large and small ; 
the large ones have a number of flat processes standing out from the sides, into each of 
which apparently a canal runs. The smaller kind of hair is narrower, and bears more 
needle-shaped processes on it. 
b. The Genital Operculum.—This is formed of a right and a left portion, which have 
fused more completely across the middle line than have the lamelliferous appendages. 
It consists of an anterior and posterior lamella, which are separate and chitinous 
right across, there being no membranous “sternal lobe” nor tongue-like appendix 
(Pl. LXXIV. figs. 4, 5). 
The posterior lamella bears no gill-book; but about one third of the way from the 
base of the appendage, and near the middle line, are a pair of small chitinous papille ; 
