TUNICATA. 315 
connected Salpa ; so different, indeed, that it might well appear to 
belong to another type. Chamisso, Krohn, and Milne-Edwards have 
ascertained that the Salpa undergoes what is called an alternation of 
generation, the young creature being unlike its immediate parent. 
One of these generations is represented by the solitary individuals, 
the other by the aggregation of individuals. Each solitary Salpa 
engenders a new form, which is the chained form; whereas each 
constituted member of the’ chain engenders a solitary Salpa, 
Thus a Salpa is not organised like its mother or daughter, but 
rather like its sister, its grandmother, or granddaughter—another 
example of alternate generation, which has already been discussed in 
treating of some of the Hydrozoa. 
These marine creatures, which pass their lives in a forced com- 
munity-—animals which eat, sleep, or rest always in company—-who 
abandon themselves together to the soft caresses of the waves—these 
colonies, or rather republics of animals, leading constantly the same 
monotonous existence—reveal to us very strange things: an identical 
community of sentiments in a crowd of beings riveted by the same 
chain, a chain at once physical and intellectual. 
