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



The Test is leathery ; it is tough and firm, but thin, and is slightly thickened at the 

 posterior end ; it is white on section, and glistening on the inner surface. 



TJie Mantle is rather thick, and is strongly muscular. It is firmly attached to the 

 inner surface of the test in most places. 



The Branchial Sac is large and moderately strong, and has four wide folds on each 

 side converging towards the oesophageal aperture. The transverse vessels are nearly all of 

 one size. Here and there, at distant intervals, a very much larger one is present. The 

 internal longitudinal bars are broad and ribbon-like ; there are six on each side of a fold, 

 and the same number in the interspace. The meshes are transversely elongated, and 

 contain each six or seven stigmata. 



The Endostyle is prominent and conspicuous, and is rather undulating in its course. 



TJie Dorsal Lamina is a plain broad membrane, with no ribs nor teeth. It extends 

 for about two-thirds of the length of the branchial sac. 



TJie Tentacles are simple; they are numerous, of several sizes, but not placed regularly. 



TJie Dorsal Tubercle is placed in a moderately large peritubercular area ; the shape is 

 cordate, elongated transversely, and with the horns simply turned in ; the aperture is 

 anterior. 



The most noteworthy feature in the external appearance of this species is the pos- 

 teriorly placed root-like processes of the test, which are attached to fragments of shells 

 and Polyzoa (PL XXIV. fig. 6) ; the longest process measures 1*5 cm. The widest part 

 of the animal is just in front of the rounded posterior end, and the sides converge rapidly 

 towards the narrow anterior end, which terminates in the branchial aperture. The surface 

 is nearly smooth, the wrinkles being very fine and closely placed ; they are present chiefly 

 on the ventral edge of the right side. There are a number of radial corrugations round 

 the apertures, but no distinct lobes. 



The thin, tough yellowish test exactly resembles that of Microcosmus. The mantle 

 is of a yellowish-brown colour, and has the musculature strongly developed. It is chiefly 

 circular. 



The folds in the branchial sac are large, and are supported by wide internal longi- 

 tudinal bars (PL XXIV. fig. 7). The occasional wide transverse vessel (tr.) equals in 

 size the height of a mesh. The stigmata have about the same breadth as the fine longi- 

 tudinal vessels and are very regular. The meshes are also very uniform and are never 

 divided transversely. 



The endostyle is prominent and conspicuous, and is rather undulating in its course 

 along the ventral edge. 



The oesophageal aperture is placed about two-thirds of the way down the dorsal edge 

 of the branchial sac. 



