ON HJ5MONAIS LAURENT II. 77 3 



the condition in the region near the budding zone soon after the first appearance of 

 the latter. The budding zone is supposed to occur after the thirty-fifth segment of the 

 original animal ; and at this stage segments 36, 37, 38 . . . of the old animal (which 

 will become vi, vii, viii ... of the new) have dorsal setae — the dorsal setae of the 

 hinder part of the original undivided animal. It is these which must fall off, if the 

 new animal (B) is to resemble the parent in having dorsal setae only from about the 

 twentieth segment onwards. 



This conclusion does not rest on inference only ; the various stages of the shedding 

 of the dorsal setae can be observed by taking individuals which are approaching, and 

 others which have lately passed, the actual division. Thus an animal was observed 

 in which the budding zone had only recently appeared (the four new anterior bundles 

 of ventral setae at' the head end of B had not begun to form) ; one bundle of dorsal 

 setae had already disappeared. In a specimen slightly more advanced (the four new 

 bundles of ventral setae beginning to show), the first, and the hair-seta but not the 

 needle-seta of the second, dorsal bundle had disappeared. Actual separation takes 

 place at about this stage, before the ventral setae of the anterior end have attained 

 their full development ; in such a specimen, after separation, two dorsal bundles and 

 the hair-seta of a third had fallen off, so that there was only a needle-seta in segment 

 viii, while the dorsal bundles were complete from ix onwards. In another, apparently 

 fairly recently separated specimen, the dorsal setae began in xi, and here again in 

 the first bundle the hair-seta had fallen out. In another specimen the dorsal setae 

 began in xiii, in another in xiv. 



The setae, too, leave traces behind them. In the specimen where setae began in 

 xi, there was present in x, at the former site of the setae, a slight epidermal thickening, 

 and, beneath this, remains of the cells of the setal sac and the setal muscle-strands. 

 In ix the traces were more doubtful, and the setal muscles had disappeared ; but in 

 viii and vii the condition was as in x ; no trace of muscles or sac could be seen in vi. 



A little consideration will show that the ventral setal bundles have also to 

 undergo alterations in connection with the separation of the posterior animal. In 

 a well-developed specimen the ventral setae are found to change their character, as 

 has been described, about the fifteenth segment. A reference to the text-figure 

 (text-fig. 4) will show, however, that the ventral setae of the posterior animal B, 

 from segment vi onwards, will at first be all of the posterior or stout type ; since 

 these segments are the 36th, 37th, 38th ... of the original animal. The stout 

 setae of segments vi-xiv must therefore fall out and be replaced by setae of the 

 slender anterior type before the zooid B can be considered fully developed. 



Indications of such a change were seen in the specimen already referred to, where 

 one bundle of dorsal setae had disappeared. Here the corresponding ventral bundle 

 contained two setae of the stout type and one of the slender, the latter presumably 

 newly formed ; the same was the case in the next segment ; while in the one following 

 this the complete setae of the stout type were accompanied by the just-formed head 



