MUSEUM OF COMPARATIVE ZOOLOGY. 199 



band of cilia. The caudal embryonic spots are arranged in a row side by side, 

 near the lower or caudal pole. From each pigment spot, cephalic or caudal, 

 pass towards the centre of the larva a number of delicate threads, which are 

 ultimately lost in the larval body. The cephalic protuberance is formed of 

 two layers, an outer transparent, and an inner more opaque. The large cells, 

 macromeres, in the interior of the larva, are smaller in number, and do not 

 occupy as large a part of the embryo as formerly. The ciliated band has nar- 

 rowed relatively to the length of the whole embryo. 



Fig. 4 is taken from a larva still older than the last. In this embryo, the 

 cephalic protuberance has been but little changed, while the caudal has grown 

 much larger than formerly. The most important additions in this embryo are 

 two rows of lateral pigment spots on the posterior margin of the ciliated band. 

 These lateral spots persist into very late stages in the growth of the worm, and 

 are identical with those collections of pigment found on the auricles which bear 

 embryonic cephalic spines in Nerine, Sjno, and other genera. The row of 

 lateral ocelli (1) cross the body of the embrj-o following the lower edge of the 

 ciliated band in an oblique direction. Around the dorsal surface of the larva 

 the ciliated band is uniform in breadth, but on the opposite or ventral side it 

 contracts and narrows to fully two thirds its former width. The oblique di- 

 rection of the lateral lines of pigment is due to this narrowing of the ciliated 

 band. Posterior to the narrowest part of the ciliated band is a clear space, 

 into which, on the ventral side, the mouth opens. 



The posterior or caudal portion, which is in reality the growing body of the 

 worm, now becomes more elongated, and the two layers which form its walls 

 become more and more e-\adent. The central part is not yet differentiated into 

 stomach and intestine, but is made up of large and small clusters of original 

 macromere cells. 



In Fig. 7 is seen the formation of the first body segment with its solitary 

 spine. The head is now more pointed than formerly, and through its walls, 

 near the upper margin of the ciliated band on the ventral side, two small 

 clusters of cells, the origin of the infraoesophageal ganglia, appear. The body 

 has become more elongated, and is swollen midway in its course, at the points 

 in which the first parapodia are formed. One or two more pigment spots have 

 formed at the posterior terminus of the body. 



The spines first appear as needle-like bodies, enclosed in the lateral walls 

 midway between the posterior border of the ciliated band and the caudal pole 

 of the embryo. It is only after they break through the walls which surround 

 them, and greatly increase their size, that their extremity is modified in the 

 manner shown in the figure. 



In Fig. 8, a still older larva than that last described, it can be seen that the 

 spines of the first parapodium have already broken through the external body 

 walls, and a second segment has begun to form behiud the first. As in the 

 former only a single spine at first appears on each side, so here there forms 

 on the second parapodium a simple spicule imbedded in the body walls. The 

 whole worm has lengthened very considerably, and a mouth has broken through 



