HOW COMPOUND ASCIDIANS BECOME ASSOCIATED. 457 



and structure of the Simple Ascidians described in 

 the last chapter. 



The Compound Ascidians during the earliest period 

 of their development are thus seen to be solitary, 

 swimming about like tadpoles in search of a suitable 

 locality whereon to fix themselves, while subsequently 

 they are found under various forms, grouped together 

 in colonies composed of very numerous individuals. 

 The next question, therefore, which presents itself for 

 solution is relative to the manner in which these 

 groups are established, and how the numerous in- 

 dividuals thus associated become arranged after a 

 pattern so decided and constant in each species. 



Various hypotheses have been framed by different 

 naturalists to account for this remarkable pheno- 

 menon. In dissecting the Botrylli (PL VIII. fig. 1), 

 M. Savigny remarked a multitude of little mem- 

 branous tubes, slightly dilated at their extremities, 

 surrounding the margins of the stellate groups ; but, 

 as his observations were confined to specimens pre- 

 served in spirits, this indefatigable observer was 

 unable to do more than indicate the existence of such 

 minute filamentary processes. Milne-Edwards, how- 

 ever, while prosecuting his examinations on living 

 animals, sufficiently transparent for the purpose, soon 

 convinced himself that each of these marginal append- 

 ages is in reality a little tubercle, or rather bud, de- 

 veloped upon the surface of the abdominal portion of 

 the adult Ascidian, which, as it becomes gradually 

 elongated, assumes the appearance of a tube having 

 its free extremity closed ; whilst, by its opposite end, 

 it remains in communication with the animal from 



