THE MOLLUSKS. 183 
of locomotion, and one which no other anima: possesses. Not long 
since a ship was constructed on this principle; its engines forced 
out jets of water from its stern! When a chain of salpas is taken 
from the water, the individuals let go their hold, and the company 
is dissolved—the soldiers no longer can form their line. Some- 
times a solitary salpa is met with. This used to be regarded as 
a distinct species, but now we know that both the parent and the 
offspring of the gregarious salpas are, at one period of their 
existence, solitary—that is, the chain salpas do not produce chain 
salpas, but solitary salpas; and these in their turn do not produce 
the solitary species, but chain salpa. Thus a salpa is not like 
its mother, but takes after its grandmother. Of course the dis- 
covery of this phenomenon has solved the difficulty which was long 
felt by naturalists, and has at once connected. the solitary and the 
associated salpas. But what patient research was expended upon 
its elucidation—research which has made eminent Milne-Edwards, 
Chamisso, and Krohn. 
In taking leave of these associated mollusca, we cannot fail 
to be struck with wonder at the grand idea upon which they 
are constructed—myriads of individuals—a crowd of beings joined 
to each other, and borne upon the waves, while from one end to 
the other of the living chain, thrills the same feeling, and the 
whole tribe is actuated by the same instinct. 
