90 THE VOYAGE OF H.M.S. CHALLENGER. 



fibrous tissue, penetrated by numerous branching anastomosing spaces (the "palli;il 

 capillaries " of some authors), and containing round, oval, fusiform, and stellate connective 

 tissue corpuscles. 



The inner surface of the mantle is lined by a continuous layer of tesselated epithelium 



—the " parietal layer of the atrial membrane " of Huxley, and the " lining membrane " 



of Hancock. The cells are squamous, polygonal in outline, and rather large. The nuclei 



are comparatively small, circular, central in position, strongly refracting, and stain deeply 



with carmine. 



The Branchial Sac is the most characteristic feature of the genus, and presents 

 striking peculiarities. In the present species it is of considerable size, occupying the 

 whole of one side of the body of the animal. To the naked eye it presents the apjaearance 

 of a coarse network, the meshes being very large and the vessels of considerable calibre. 



A closer examination shows that, as in all the Cynthiidae, the sac presents certain 

 folds running longitudinally (or from the branchial to the oesophageal aperture) and 

 projecting from its internal surface. These folds are twelve in number, six on each 

 side of the sac. They are of moderate size, not very prominent, but still distinctly 

 visible. Those nearest to the dorsal edge of the sac are more distinct, and are more 

 closely placed than those towards the ventral edge, the pan- next the endostyle being 

 very slight. 



The structure of this branchial sac is simple in the extreme (PL VIII. fig. 3). 

 There are two series of vessels — the transverse and the internal longitudinal bars. If 

 the branchial aperture be placed superiorly the transverse vessels will be found running 

 round the sac externally like a series of horizontally placed hoops, while the internal 

 longitudinal bars lie in a plane internal to the transverse vessels, and run down the 

 inner surface of the sac from the anterior to the posterior end. The two series of vessels 

 thus cross at right angles and foim a network with rectangular meshes. At their j)oints 

 of intersection — the angles of the meshes — the vessels intercommunicate. 



The transverse vessels (PI. VIII. fig. 3, tr. and trf) are of two kinds, larger and 

 smaller ; they are placed alternately, and the larger vessels are about three times as wide 

 as the smaller ones. The meshes formed by the intersecting transverse and internal longi- 

 tudinal vessels are oblong, and have their greater extent antero-posteriorly or at right 

 angles to the transverse vessels. Over the greater part of the branchial sac, the propor- 

 tion between the sides of the mesh varies from 3 : 4 to 3:6 and is in most cases about 

 3:5, so that the transverse vessels are placed from once and a-half to twice as far from 

 one another as the internal longitudinal bars are. On the folds, however, the meshes are 

 much more elongated, in consequence of the comparative crowding together of the internal 

 longitudinal bars on these parts, while the transverse vessels are at the same distances as 

 in other regions of the sac. 



The folds (PI. VIII. fig. 3, br.f.) are merely longitudinal tracts along which the 



