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



specimens differ somewhat in the position of the viscera (PI. X., compare figs. 1 and 9). 

 The visceral mass forms a compact rounded clump, part of which is coloured dark 

 brown, while the remainder is pale yellow (PI. X. fig. 9). The browTi portion is the 

 alimentary canal, and the yellow is formed of the reproductive organs, part of it being 

 the ovary and the rest the cajca of the testis. Fig. 10 shows the anterior surface 

 of the visceral mass on which the nervous system {n.g.) is placed. The large dark- 

 coloured mass is the wall of the stomach (st.), and the reproductive organs appear 

 at both sides of it. 



On the whole, I regard this form as being allied to Salpa. The condition of the 

 visceral mass is very like the " nucleus " in Salpa, and occupies much the same 

 position in the Ijody (compare Fig. 7, p. 55, and Fig. 11, p. 93). The musculature might 

 readily be derived from a series of transversely-running bands l:)y an antero-posterior 

 shortening which would approximate the bands closely, and then by a portion of the 

 muscles l)eing drawn out radially into the eight conical processes. The endostyle and 

 the nervous system are in their proper places, but there seems to be no trace of a 

 dorsal lamina ; and the branchial sac is certainly in a remarkable condition. If the 

 obliteration of the side walls of the sac in Salpa has been Ijrought about by the 

 locomotory habits of that form, of course no such change would be necessary in the 

 case of an ally such as Octacnemus, which was attached, or at least not locomotor ; 

 but it is difficult to see why the stigmata in the walls of the sac should become closed 

 up, unless perhaps nutrition and aeration were performed sufficiently by the water 

 gaining access to the large cavities of the body through the branchial and atrial 

 apertures, without there being any definite current. 



Order III. LARVACEA. 



The Larvacea are free-swimming pelagic forms, provided with a large locomotor 

 appendage (the tail), in which there is a skeletal axis (the urochord). 



A relatively large test (the "Haus") is formed with great rapidity as a secretion 

 from the ectoderm ; it is merely a temporary structure which may be cast oft' and 

 replaced by another. 



The branchial sac is simply an enlarged pharynx with two ventral ciliated openings 

 (stigmata) leading to the exterior. These open independently on the ventral surface. 

 There is no separate peribranchial cavity. 



The nervous system consists of a large anterior and dorsally-placed ganglion, and 

 a long nerve -cord, with smaller ganglia, stretching backwards from it over the 

 alimentary canal to reach the tail, along which it runs on the left side of the 

 urochord. 



