No. 3.] THE CELL-LINEAGE OF NEREIS. 423 



setae appear in the two anterior pairs, and the paratroch {par.) 

 appears around the margin of the pigment-area. 



The setae are protruded from the two anterior pairs of seta- 

 sacs about the fortieth hour (Fig. 89), the others not until a 

 considerably later period. This is followed by the appearance 

 of the parapodial cirrhi in the second and third somites (they 

 are never developed in the first), and at the same time two 

 incomplete mesotrochal bands of cilia appear (Figs. 90, 91) at 

 the posterior margin of the first and second somites respectively. 



From this time forwards the body rapidly develops into the 

 adult condition. About the sixtieth hour (Fig. 91) the anal 

 cirrhi {a.c}j make their appearance inside the paratrochal ring ; 

 at the same time the cephalic tentacular cirrhi (f.c^ appear, 

 posterior to the prototrocJi, two new pairs of eye-spots are devel- 

 oped, and the setas of the third somite are put forth. The 

 somites become distinctly marked off, the parapodia become 

 prominent, and the larva now possesses a distinct head and a 

 trunk consisting of three somites. 



Certain other details in the larval development demand a brief 

 mention. Towards the fortieth hour (or earlier) a granular 

 reddish-brown pigment appears in the cells adjoining the large 

 prototrochal cells, and to some extent in the peripheral portions 

 of the ciliated cells themselves. The large ciliated cells are 

 thus rendered extremely conspicuous (Figs. 89, 90), and may 

 easily be counted in the living larvae. The pigment is very 

 irregularly distributed, and varies extremely in different individ- 

 uals. It is moreover changeable in the same individual, for a 

 sudden disturbance seems to cause a contraction either of the 

 pigment-cells or of the pigment-granules (chromatophores .''). I 

 have often observed finely pigmented specimens swimming in 

 a watch-glass, which when brought under a cover-glass (still 

 actively swimming) at once became nearly colorless. As the 

 animal grows older, the girdle of pigment becomes broken up 

 into irregular patches, two of which, one on either side the 

 head, usually persist until a late stage (cf. Fig. 92). Ultimately, 

 however (fifteen days), the pigment disappears. 



About the sixtieth hour (Fig. 90) a number of peculiar sense- 

 hairs appear at the anterior and posterior extremities. Ante- 

 riorly there is a group of about seven short, stiff, knobbed hairs 

 on the apical region (Fig. 90) (the apical cilia have disappeared). 



