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



The Ted is not thick, but is tough on the upper part ; it is thinner below, except at 

 the posterior end, where it is considerably thickened. 



Tlw Mantle is thin and the musculature very delicate. 



TJie Branchial Sac has four folds upon each side ; these scarcely project, and are 

 formed merely by a crowding together of the internal longitudinal bars, six to nine being 

 placed close together, and separated by wide spaces containing only three bars each. The 

 meshes are elongated antero-posteriorly, contain each only three stigmata, and are each 

 divided transversely by a narrow membrane. 



The Dorsal Lamina is narrow and much crumpled, but is neither ribbed nor toothed. 



Tlie Tentacles are simple and rather large, there are twelve of them. 



The Dorsal Tubercle is rather prominent, it is cup-shaped, with a wide anterior opening. 



This species was probably buried in sand nearly half its length. The posterior 

 part of the test is of a different texture from the anterior, has a different appearance 

 (PL XX. fig. 7), and is incrusted here and there with sand. The apertures are both 

 sessde, fourdobed, and distinctly visible at the anterior extremity. The test is thin, 

 leathery and tough, as in the typical Cynthiidse. At the posterior end, it is considerably 

 thickened. 



The branchial sac has the folds in a rudimentary condition. They are no longer true 

 folds and do not project into the cavity of the sac, but are merely longitudinal tracts 

 along which there is a crowding together of the internal longitudinal bars (PL XX. 

 fig. 8. l>r.f.). Consequently, the meshes on the inner surface of this branchial sac are 

 of two sizes, — (1), the ordinary ones, which are nearly scpiare and contain each usually 

 three stigmata ; and (2), the very much narrower ones, half a dozen rows of which 

 represent the branchial fold. These last contain one or sometimes only part of a stigma. 

 The transverse vessels are of two sizes placed alternately. The narrow horizontal mem- 

 branes crossing the meshes are only present occasionally, and sometimes run for very 

 short distances only (PL XX. fig. 8). 



In this sac there are a considerable number of muscidar fibres, which are present not 

 only in the large transverse vessels, but also in the fine longitudinal mterstigmatic 

 vessels (PL XX. fig. 9, m.f.). The dorsal lamina is much crumpled, but has a smooth 

 edge. 



One specimen of this species was dredged in the South Atlantic (off the coast 

 of Buenos Ayres), at Station 320 ; February 14, 1876 ; lat. 37° 17' S., long. 53° 52' W.; 

 depth, 600 fathoms; bottom temperature, 2° 7 C; bottom, hard ground. 



Styela flava, Herdman (PL XX. figs. 1-6). 



Styela flava, Herdman, Prelim. Rep., Proe. Roy. Soe. Edin., 1880-81, p. 64. 

 External Appearance. — The body is rudely spherical, but is slightly elongated later- 



