ESTIVATION OF BOTRYLLOIDES GASCOI BELLA VALLE. 155 



The vascular system seemed to be the mechanism chiefly concerned with the 

 growth of any region of the colony. The ampullae were always at the edge of the 

 growing region, secreting the test substance and pushed forwards by the constant 

 elongation of the vascular stalks. Even in the case of the buds and zooids I think 

 that the motive power was also the elongation of the connecting vessels. It may 

 be that the buds and zooids had some power of locomotion by means of their ecto- 

 derm cells apart from the influence of their connecting vessels; and it is hard to explain 

 the formation of systems in Botryllus without assuming some such power. But in 

 Botrylloides all the growth seemed easily explicable as a result of the vessels alone, 

 for there were always one or more vessels connected with the buds, and the growth 

 of the vessels would tend to push the buds forwards. Furthermore, these vessels 

 were usually attached to the posterior part of the bud which progressed on 

 this account (as has been shown above) with its anterior or branchial end in 

 advance. 



2. Lack of Coordination in Budding and in the Formation of Systems. In the pre- 

 ceding section it has been shown that there was considerable variation in the 

 times of appearance and disappearance of the buds and zooids. This subject will 

 now be taken up somewhat more in detail. 



The most striking feature of the activities of the Botryllidse, which was already 

 known to Krohn ('69, '69 a ) in the case of Botryllus, is the exactness of the coordi- 

 nation, between zooids of the same generation, in budding and in the formation of 

 systems. Thus there is only one generation of adult zooids present in the colony at 

 any one time, and all the members of this generation are of the same age and their 

 buds and embryos are of the same age. All of the zooids of this generation degen- 

 erate together and leave similar buds to replace them. When these buds, by opening 

 their siphons to the exterior, first assume the functions of adult zooids they are not 

 arranged in the circular or oval systems, each with a common cloacal orifice, char- 

 acteristic of Botryllus, but at this stage each young zooid has a separate cloacal as 

 well as a separate branchial orifice, and it is only by a rearrangement of these that 

 the new systems are formed. Thus these new systems have no genetic continuity 

 with the old systems, and Krohn stated correctly that the buds produced by the 

 zooids of one system may form a single system, or separate into two groups to form 

 two systems; or the buds from zooids belonging to two systems may unite to form 

 a single one. 



These results have been abundantly confirmed by later investigators, especially 

 by Pizon ('99, : 00), who has also worked with living colonies and found that these facts 

 held for Botrylloides rubrum as well. All of my observations on living colonies of 



