THE DEVELOPMENT OF BOTRYLLUS. 527 



plete, a zooid is formed from it, so that, at one time, the em- 

 bryo appears to have two branchial sacs. 



Metschnikoff ' and Krohn'^ have shown that the caudate 

 larvae of Botryllus are not composite, as Savigny and Sars 

 supposed, but that the bodies imagined by these observers to 

 be buds are simply diverticula of the ectoderm, and become 

 converted into the vascular processes, which ramify through 

 the common test, and commonly end in dilatations. In the 

 adult, the buds are developed, one, or sometimes two, at a 

 time, at the sides of the body, and consist of an outer layer, 

 derived from the ectoderm, and an inner layer, which, accord- 

 ing to Metschnikoff, proceeds from the atrial tunic. From 

 the inner layer the alhuentary canal of the bud proceeds, and 

 between the inner and the outer layers the rudiments of the 

 genitalia appear. The ovaria advance toward their develop- 

 ment much more rapidly than the testes. The zooids thus 

 developed, as they enlarge, rise to the surface, taking the 

 place of those from which they proceed and which die away 

 The ova are impregnated from without, and undergo their de 

 velopment in the atrium of the parent. Subsequently the 

 testes attain their full development ; and, at the same time, 

 the buds are formed which will give rise to a third generation 

 supplanting the second. 



After the larva (which may be called A) has attached it 

 self, the first sets of zooids which are developed are sexless 

 The first bud arises on the right side of the body of the larva 

 (A) in the neighborhood of the heart ; as it increases in size, 

 the parent withers away, and the zooid (B) thus developed 

 takes its place. Two buds, a right and a left, are developed 

 from (B) and become zooids (C, C), B disappearing. The two 

 zooids (C, C) are so disposed that their atrial ends are close 

 together, and their oral ends turned away from one another. 

 These each develop two lateral buds, which become four 

 zooids (D, D, D, D). The zooids (C, C) disappear as before, and 

 their successors arrange themselves in a circle. Each of these 

 develops two, or sometimes three, lateral buds ; these grow 

 into zooids, which supplant their predecessors, and are them- 

 selves, in turn, supplanted. 



Every new system of the later successions is, at first, de- 



1 " Entwickeluiiffsoreschichtliche Beitrage." ("Bulletm de I'Academie des 

 Sciences de St.-PA.ersbourg'," xiii, 18GR). 



2"Ueber die Fortpflanzuiigsverlialtnisse bei den Botrvlliden" ("Archiv 

 fiir Naturgeschiclite," 1809). " Ueber die friihesto Bildung der Botrvllen- 

 •tocke" {ibid.). 



34 



