SWIMMING ASCIDIANS. 319 
manner, which would seem to make them compo- 
site animals, though they are not such; and he 
divides it into two tribes:—1. The simple Sal- 
paceans (Salpa); 2. The aggregate Salpaceans 
(Pyrosoma). . 
‘A great interest,’ observe Messrs. Forbes and 
Hanley, “is attached to the natural history of the 
Salpe, on account of their singular mode of repro- 
duction, discovered by the German naturalist 
Chamisso, and the extraordinary generalization to 
which that discovery in a great measure gave rise. 
Previous observers had noticed that these animals 
were sometimes found solitary, at others united 
together in long chains, composed of numerous 
individuals of similar form, each an independent 
being, though constantly associated, and linearly 
ageregated with its companions. These long 
chains swim through the tranquil water with 
regular serpentine movements; for the creatures 
of which they are composed contratt and expand 
simultaneously, keeping time, as it were, like a 
regiment of soldiers upon parade. Each chain 
seems, consequently, to be a single being, acting 
through the influence of an unique will; and hence 
sailors often look upon it as a reptile; and in many 
seas the Salpa-chains are called sea-serpents. But 
when taken out of the water, the links of the chain 
fall asunder, the several distinct animals of which 
it is composed suddenly losing their power of ad- 
hesion. In consequence of accidents, broken-up 
chains and separated members of such communities 
are not unfrequently met with, in seas where Salpe 
are numerous. But other Salpe are also met with, 
very dissimilar in form, and never united together 
in chains. Now, the discovery of Chamisso was, 
