On the Life-History and Development of the Genus Myzostoma. 557 



on the surface of the praeoral lobe are planted a number of long fine, 

 stiff cilia whieh are probably processes of the larvai nervoiis cells, and 

 hence may be regarded as larvai sense org-ans. All these things are 

 figured in figs. 19, 20, 21, 22. During the following days, from the 

 third to the seventh the development eonsists merely in an inerease in 

 size of the larva and in the further differenti ation of the organs already 

 present. 



It might a priori be expeeted that on the third day or siibse- 

 quently traces of segmentai organs would be formed. This is however 

 not the case. In spite of very mueh time and trouble spent in attempts 

 to find such organs the result at all stages of the larvai history has been 

 piirely negative. I can only conclude that in Myzostoma segmental 

 organs are not formed during the larvai development. This absence of 

 head kidneys cannot I think be considered as reason for shutting Myzo- 

 stoma out of the group of Chaetopoda, for judging by the observations of 

 Gotte it shares this negative character with Nereis Dtmierilii^ at least 

 in bis work segmental organs are neither mentioned in the text nor 

 figured in the plates. Further at the time of writing this so little is 

 published concerning the occurrence of larvai segmental organs among 

 the Chaetopoda that failure to find them in the larva of Myzostoma need 

 occasion little concern. In figs. 21, 22, 23 and 24 larvae from the fourth 

 to the seventh day after fertilisation are figured. The main difiference 

 they show from three days larvae eonsists in inerease in size. 



The larva can now move about in search of food. Its nervous system 

 has reached a fair development, and is well marked in every individuai 

 (figs. 23, 24, n.s.l). In fig. 24 especially it is a prominent feature. 

 This figure and fig. 23 show too a ventral thickening of the epiblast, 

 which has now formed. This is the foundation of the future ventral 

 ganglionic mass. The sense hairs on the praeoral lobe have not grown 

 and remain in much the sanie condition as on the third day figs. 21, 

 22, 23, 24, sJi). 



In fig. 24 is also seen a muscle cord, which like that described by 

 Hatschek 1 in Polygordius passes from the apex of the praeoral lobe to 

 the head end of the larva. 



The alimentary canal is now very fully developed, and has ac- 

 quired the foundation of the future proboscis. This latter is at this stage 

 not protrusible but is very muscular. It has been formed by the develop- 



1 Hatschek, Studien über Entwìekl. der Anneliden. Arbeiten a. d. Zool. 

 Institut zu Wien. Bd. I. Heft III. p. 31. 



