320 MR. W. B. S. BENHAM ON THE MUSCULAR 



cavity above ; the sides of this bag may be termed the anterior and posterior lamellae. 

 Across the middle line, for a short distance on each side, these two lamellae 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 (PI. LXXI1I. fig. 4, ml), 

 containing a space continuous with that between the lamellae. 



A single branchiferous appendage, considered apart from its fellow to which it is 

 joined across the middle line, consists of a broad, fiat, 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 PI. LXXIII. 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 lamellae 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 lamellae are also one or two 

 small sclerites (see PL LXXIII. fig. 4). 



Close to the base of the sternal lobe, on each side, and close to the middle line, is 

 situated a stigma (stg) ; this leads into a hollow tendon, which passes upwards and 

 forwards for about f 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 lamellae 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 

 (PI. 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 papillae ; 



