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



Molgula forbesi, Herdnian (PL V. figs. 8-11). 



Molgula forbesi, Herdnian, Preliminary Eeport, Proc. Roy. Soc. Edin., 1880-81, p. 23C. 



External Appearance. — The body is globular in shape, slightly elongated dorso- 

 ventrally, and not compressed laterally. The anterior end is slightly projecting, narrow, 

 flattened, and has a siphon at each extremity. The posterior end is wide, and regularly 

 rounded. The dorsal and ventral edges are both extremely convex ; the body is not 

 attached. The apertures are both at the anterior end, they are not distant, but are slightly 

 prominent. The branchial is at the ventral edge of the anterior end, and is turned 

 ventrally; the atrial is at the dorsal edge, and points anteriorly, it is more prominent 

 than the branchial. 



The surface is entirely covered with a close coating of sand grains. The colour is 

 dull brown (due to the sand). 



Length of the body, l - 8 cm. ; breadth of the body, 2 cm. 



The Test is not thick but stiff ; it is quite opaque. 



The Mantle is thin and transparent, the viscera being seen through distinctly. The 

 muscle bands are numerous, but very fine. There are a few stronger circular bands 

 round the short tubular siphons, and a series of longitudinal bundles radiate from the 

 base of each siphon. 



TJie Branchial Sac is rather delicate, and has seven folds upon each side. The 

 transverse vessels vary greatly in calibre and position, and are often quite irregular. 

 The internal longitudinal bars are strong, and there are three or four on each fold. The 

 stigmata are very irregular, being straight, and arranged in transverse rows in some 

 places, while in others they are curved, and form spirals, or run irregularly. Delicate 

 horizontal membranes are frequently present, running from fold to fold. There are also 

 more irregular longitudinal and oblique membranes. 



The Dorsal Lamina is a short and narrow membrane with a plain edge. 



The Tentacles are compound, situated on a strong muscle band. There are about 

 twelve large and twelve small placed alternately. 



The Dorsal Tubercle is simple, and placed at the posterior end of a deep and irregular 

 peritubercular area. The left side of the tubercle extends further anteriorly than the 

 right ; both horns are turned to the left. 



I have dedicated this elegant little species to Professor Edward Forbes who founded 

 the genus Molgula. It has a globular shape (PI. V. fig. 8) with the apertures at the 

 anterior end, forming what are evidently permanent projections, since the sandy coating 

 extends over them up to the very edge of the apertures. 



The mantle is very delicate and quite transparent, allowing the viscera to be seen 

 through distinctly (PI. V. fig. 9). The branchial sac is also delicate, and the folds 



