Gatty Marine Lahoratory, St. Andrews. 255 



the lip-organ {Ip.) appears. A pale band indicates a differ- 

 entiation outside the cellular layer dorsally and another 

 laterally. The chamber by-and-by assumes a figure-of-8 

 outline, the section of the lip-organ occurring in the lower 

 division; and this shows a dorsal lenticular region of firm 

 pale nucleated cells, somewhat symmetrically arranged, the 

 lower part still having its cavity surrounded by the softer 

 and more deeply-stained nucleated cells of the vestibule 

 (PI. XI. fig. 26). The figure-of-8 outline of the chamber 

 is now complicated by a median process on each side and by 

 the appearance of a diverticulum (gullet) dorsally, whilst the 

 increase in its size diminishes the space between it and the 

 body-wall laterally and superiorly, though from the first it 

 clings to that wall ventrally. The pale streaked dorsal region 

 of the lip-organ is gradually increasing in size as the sections 

 pass backward ; the diverticulum joins the upper region of 

 the canal, which is soon separated from the lower by the 

 junction of the median processes or isthmuses, thus con- 

 fining the lip-organ to a special chamber (PI. XII. fig. 27). 

 The»upper chamber is lined by the soft cellular mucous 

 coat; the lower has a thin lining of epithelium, with longi- 

 tudinal and a few circular muscular fibres externally, the 

 whole becoming continuous with the upper edge of the lip- 

 organ on each side, the remains of the ventral wall with its 

 mucous lining at first linking it to the lip- organ and then 

 disappearing, the mid-ventral region being occupied by 

 strong muscular fibres, probably the protractor of the 

 organ. The nerve-cords are on each side and widely 

 separated, and the ventral wall of the body is very thin. 

 A blood-vessel lies on each side at the tipper edge of the 

 lower chamber; a section of a succeeding part of the canal 

 appears at the upper border of the wall of the lower chamber, 

 and soon stretches across the entire region. A change 

 is also taking place in the upper chamber, the lower region 

 of which is thickened and its cells rendered paler. In the 

 roof of the lower chamber the cells are assuming the chordoid 

 condition of those in the lower region, so that very soon both 

 halves make an efficient organ for compression or manipu- 

 lation. The upper chamber becomes also smaller, and the 

 space between the two larger. Strong mviscular fascicules 

 appear both dorsally and ventrally over the lingual organ in 

 the lower chamber ; and the nuclei in the elongated cells of 

 the modified organ form a row nearer the outer than the 

 inner border. In the interval between the upper and lower 

 chambers another diverticulum of the upper chamber has 



