ZooL— Vol. III.] BANCROFT— COMPOUND ASCIDIANS. 1 77 



individual and not as an aggregation of zooids; in other 

 words, what characters has it that cannot be considered as 

 the sum of the characters of its zooids? 



This question is obviously very different from the first 

 one. It deals with the morphological and physiological 

 individual, and is not concerned with names but with the 

 extent to which the parts have merged their individuality 

 into that of the whole which they compose. 



As indicated above, the individuality of the Botryllus 

 coJony may be spoken of as manifested in both a morpho- 

 logical and a physiological way, though it must not be 

 forgotten that every character can be considered from a 

 morphological or physiological point of view. 



The morphological characters are: — 



1. Separate localized existence. 



2. Investment of the zooids in a common test. 



3. Presence of a common vascular system. 



4. Characteristic and comparatively constant colors of 

 the zooids. 



The physiological characters are : — 



1. The quality determining whether the colony shall fuse 

 or compete with a neighboring one. 



2. The force regulating the formation and size of the 

 systems (see p. 173). 



3. The coordination in the growth and reproduction of 

 the zooids. 



4. The "force" causing the limited existence of the 

 zooids, which results in the fact that certain morphological 

 elements of the colony, such as the blood-vessels and even 

 the ova^ in the zooids themselves, have a much longer life 

 than the zooids. 



Of the morj^hological characters only the last, and that in 

 a rather doubtful manner, can in any sense be considered 



1 It has been shown by Delia Valle (1881) that the ova do not complete their develop- 

 ment in the zooid in which they originate but migrate bodily to the buds developed by 

 this blastozooid. The writer has been able to confirm this observation on living material; 

 the same ova which on one day were seen in the mother zooid, on the next day were 

 seen to be absent from this zooid and present within the buds which it had developed. 



