HISTORY OP THE SALP^E. 



233 



young becomes detached at an early age, and continues 

 to increase in bulk, and reaches maturity by its un- 

 assisted powers. 



Perhaps a more extraordinary instance of " alterna- 

 tion of generation," and one in which the idea con- 

 veyed in that term seems to be most fully brought out, 

 occurs in the genus Salpa, one of the Ascidians. In a 

 former chapter I noticed 

 some of the more com- 

 mon forms of this family, 

 some of which, it will be 

 remembered, are simple 

 animals ; others com- 

 pound, or living in in- 

 dissoluble association, or- 

 ganically connected one 

 with another. Now the 

 Salpa is a genus of this 

 family which, in alternate 

 generations, exhibits the 

 character of a simple or of a compound Ascidian. That 

 is to say, a compound Salpa produces simple young, 

 and a simple Salpa compound young. The nature of 

 this change will be more evident when I have described 

 the appearance of the animal in both phases of exist- 

 ence. The Salpce are at all times free, swimming from 

 place to place, and generally in flocks, through the 

 waters of the ocean. Each animal resembles a tube ; 

 clear as crystal, through whose walls the coloured inter- 

 nal parts may be distinctly seen. Sometimes these 

 animals are found solitary; at other times linked 



