DEVELOPMENT OF EMBEYO. 



485 



creature, in size not yet larger than the head of the smallest pin, fixes 

 itself to some foreign object by means of one of the little suckers 

 situated on the anterior extremity of its body, and permanently loses 

 all capability of locomotion. 



(1273.) Having thus fixed itself for life, the larval Ascidian soon 

 begins to change its form (fig. 245, B). The anterior extremity of 

 its body becomes expanded, the tentacular appendages disappear, the 

 central portion of the tail becomes retracted into the central mass, 



Fig. 245. 



Larva and progressive stages of metamorphosis in an Ascidian Mollusk. 



and, lastly, the tail itself, which was at first such an important loco- 

 motive agent, gradually withers away, until no traces remain of such 

 an organ having existed (fig. 245, c, D). 



(1274.) From the above description of the development of the young 

 Ascidians, it appears that during the first part of their existence they 

 are solitary and isolated animals, although at a later period they are 

 found united into numerous colonies, either connected together by 

 means of a creeping common stem, or associated into a compact mass 

 by a tegumentary tissue wherein the entire colony is arranged after a 

 certain order, or regular pattern, which is constant in each species ; and 

 the manner in which this is effected thus presents itself as a problem 

 possessing considerable interest. 



(1275.) Savigny, while prosecuting his dissections of the Botrylli, 

 had remarked, situated upon the borders of the stellate groups formed 

 by the association of individuals belonging to that genus, a multitude 

 of membranous tubes, slightly dilated at their extremities, to which he 

 gave the name of the marginal tubes, at the same time pointing out their 

 existence in other families, but without entering into any details con- 

 cerning the relations existing between them and the associated Ascidians 

 contained in the tegumentary mass. Milne-Edwards, however, ascer- 

 tained, by the examination of transparent groups of these creatures 

 whilst in a living state, that each of these canals is at first a little 

 tubercle, developed from the surface of the abdominal portion of the 



