556 John Beard 



acqiiires provisioual setae. These begin to arise towards the end of 

 the second day (fig. 17). They probably arise in epiblastic sacs. Ou 

 each side of the body and at some distance behind the mouth a pair of 

 chitinous rods are developed. The two setae on each side are formed 

 close together and at the same time. They grow rapidly and when the 

 larva is three days old they are already of fairly considerable size 

 (figs. 19, 20). But they bave not then attained their full grò wth, but 

 continue to grow until the larva is about seven days old (figs. 21, 22). 

 At this time they are very long, even longer than the larva itself. 



Subsequent to the third day the setae are usually iucreased in 

 number, but there is never in the larva a segmentally arranged series 

 of setae, such as Gotte ^ described and figured for Nereis Dumerilii. 

 A rough sketch of Götte's fig. 19, Piate VI I bave given on fig. 29. 

 With the formation of the provisionai setae the development of muscles 

 to move them becomes necessary. These latter appear to be formed by 

 the proliferation of part of the prìmary mesoblast. So that on each side 

 of the body muscle-cells are given off from the mesoblast, and these 

 become attached to and lie in close connection with the inner wall of 

 the setiferous sacs (figs. 27, 28). 



At this stage the thickening of the epiblast of the frontal lobe also 

 begins. In other words the larvai nervous system begins to be formed. 

 When these things bave taken place we bave a larva such as is repre- 

 sented in figs. 19, 20 and 21. Fig. 19 represents a larva oi M.glabrum 

 of 4 days and fig. 20 one of M. cirrifenmi in the same stage of develop- 

 ment. The two figures are given so as to admit of the comparison of 

 the larvae of both species. A comparison which readily leads to the 

 conclusion that at this stage the development is practically the same in 

 both species. 



The larva of three days is in a condition to provide for its own 

 nutrition, and so is pretty well fitted up for a free existence. It pos- 

 sesses fully formed mouth, alimentary canal and anus. It has two rings 

 of cilia, one immediatety behind the mouth to provide its nutrition, and 

 one in front of the anus to facilitate its locomotion. In addition it has 

 other locomotive organs in the provisionai setae, which are well pro- 

 vided with muscles, and subserve the double purpose of locomotion and 

 protection. 



Further the larvai nervous system is pretty well developed, and 



1 GOTTE, AbhandhiDgen zur Entwicklungsgeschichte der Thiere. 1. Heft. 

 Taf. VI Fig. 19, 20. 



