Gatty Marine Lahoratory, St. Andrews. 7 



these corresponded to the first region of the body, con- 

 sequently only five pairs of dorsal hook-rows are present. 

 Tlie seventh pair of bristle-tufts is slender and small, so that 

 tlie outline is narrowed, the adjoiuinjj; tufts being longer. 

 Toward the tip of the tail elongated, slender, simple bristles 

 occur as in the adult. The bristles correspond in arrange- 

 ment and structure with those of the adult. The anterior 

 hooks differ in having a shorter posterior basal process, but 

 they are accompanied by the same paddle-shaped bristles. 



The anal segment is bilobcd, and has a peculiar series of 

 dark pigment-specks. Four pairs of setigcrous processes 

 bearing short bristles occur behind the last hooks, which 

 are small and only three in number. The bristles increase 

 in length at the sixth from the tip. The opaque glandular 

 tissue splits at the termination of the rows of dorsal hooks 

 at the second ring from the latter, since the first has a bar 

 obliquely bevelled at the lower edge. The splitting con- 

 tinues to the tip of the tail and gives a regular arrangement 

 to the parts. 



Both anteriorly and posteriorly the bristles commence 

 before the hooks. Four pairs of bristles occur before hooks 

 appear, the first hooks being between the fourth and fifth 

 bristle-tufts, and tlieir bases are undeveloped. The first 

 five pairs of bristles are short, but the sixth are decidedly 

 longL-r. 



The tube may reach a length of 2 feet, and is fixed to a 

 stone or otiier structure. 



No form has attracted greater interest than the second 

 species, viz., Potumitla retiifurmis, Leuckart, the Sahella 

 sujcicava of De Quatrefages — which abounds amongst cal- 

 cai-eous formations^ such as Cellepara, Lithuthamnion, the 

 Balaiii of the Gouliot caves of Sark and elsewhere, and is of 

 special interest in connection with its power of perforating 

 such structures. 



The cephalic plate, when the branchiae are shed, presents 

 dorsally a bilobed collar or lamella, the deep dorsal furrow 

 terminating in the centre. The outer edge of each flap is 

 continued as a broad rim nearly to the mid-ventral line, 

 where a notch sepai'ates the two sides, which curve forward. 

 The truncated surface has a projecting transverse fold at the 

 upper end of the ventral incurvation, and two folds meet 

 above it — so as to make a triradiate aperture. 



The branchiae are about 10 or 11 in number on each side, 

 and comparatively short, whilst the pinnae are long. Each 

 filament has the trausverselv barred or camcrated chordoid 



