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



shape. No nuclei are distinguishable, but the cell contents are throughout brown and 

 granular. A finely granular matter, in some places traversed by very delicate strings or 

 fibrillar, lay along the floor of the groove, covering the tops of the cells ; but nothing of 

 the nature of cilia could be detected in any part. 



The Dorsal Lamina is replaced by a series of languets or tentacular processes (PL 

 VIII. fig. 8, I.), which are disposed in a single line along the dorsal edge of the branchial 

 sac from the apex of the peritubercular area anteriorly to the oesophageal opening 

 posteriorly. 



The languets are relatively long and are very closely placed (PI. IX. fig. 14, I.). Each 

 has an elongated triangular form tapering from the base where it is attached to the pointed 

 free end. They are about 4 mm. long, and are very delicate and transparent, except 

 along the lateral edges and the tip, where there is a thickened border. 



The histological structure of the languets is exactly similar to that of the internal 

 longitudinal bars of the branchial sac. They are hollow, and the thin walls are covered 

 in the greater part of their extent by squamous epithelium, which becomes thickened 

 towards the edges and the tip, where the cells are cubical, thus forming the darker border. 

 These cubical cells do not bear cilia. 



The Tentacles are large, much branched, and of various sizes (PI. VIII. fig. 7, tn. and 

 tn.'). They spring from the upper margin of the praebranchial zone, and just at their 

 bases a strong muscular band, forming the most posterior part of the sphincter muscle, 

 runs round the lower end of the branchial siphon. There are sixteen principal tentacles, 

 eight larger and eight smaller, placed alternately; but between these there are others here 

 and there of a very much smaller size and having no definite arrangement. A mode- 

 rately sized member of the circle of eight larger tentacles (PL VIII. fig. 7, tn.) is about 

 12 mm. long, and has from twelve to twenty branches. Some of these branches are 

 simple, while others, generally about the centre of the tentacle, bear simple lateral pro- 

 cesses. The eight smaller tentacles (PL IX. fig. 12) are one-third to one-half of the size 

 of the larger ones, and have generally eight to twelve processes. 



The main axis of each tentacle is an elongated tapering sac with thin walls, and has 

 a ridge or dark band running out from the base to the tip along the upper surface 

 (PL IX. fig. 12), wdiile the lower or branchial surface is soft and membranous. The 

 pinnae start off from the sides of the main axis rather towards the under than the upper 

 surface, and each of them has along its upper edge a dark ridge, while the lower surface 

 is membranous and plain as on the main axis. This membranous lower surface is also, 

 both on axis and branches, irregularly puffed out, or thrown into a series of projections 

 and folds, while the upper surface is straighter, and has the appearance of being more 

 tightly stretched. 



The surface of the tentacles is covered with epithelium, thin and tesselated over the 

 greater part of the surface, thicker and columnar along the dark band on the upper edge 



