THE MOLLUSKS. 181 
vagary of nature —animals who eat separately, but who fulfil 
together as a community very singular functions. This is a kind 
of union of which we have no other example ; the entire star may 
be considered as one single animal with many mouths. 
While the Botrylla is fixed, the Pyrosoma is perfectly free. 
This aggregation constitutes a brilliantly-coloured mass. Its form 
is that of a hollow cylinder, closed at one end. The creature 
floats and balances itself upon the water like the sea-pen. One 
of its species, the A /antica, changes its colours like the chameleon, 
only much more rapidly. It quickly passes through the shades 
which intervene between bright red and bright yellow, and on 
as quickly to golden orange, and through green to azure blue. 
Moreover, to add to its charms, it is phosphorescent. Its name, 
pyrosoma, literally signifies a “body of fire.” Humboldt saw a 
shoal of these brilliant living colonies floating by the side of his 
ship, and giving out circles of light not less in diameter than 
twenty inches. By the light of these lamps shining in the watery 
firmament, he was enabled to see the fish which swam in the 
vessel’s wake down to a depth of three or four fathoms. Bibra, 
a Brazilian navigator, having caught seven or eight of these 
Pyrosoma Atlantica, took them into his cabin, and by their light 
was able to read to his friend the account he had written of 
these light-giving mollusks. 
The Biphora, or Salpa, constitutes a group which bears a great 
resemblance to the polypier. These creatures are rather disposed 
in a series, than in an actual agglomeration; indeed, they are the 
transition group between the gregaréous mollusks we have been 
considering, and the so/itary mollusk. The salpas are long, trans- 
parent threads of extremely delicate tissue, composed of rows of 
individuals placed side by side, and grafted, as it were, transversely. 
This order of arrangement is not invariable, for sometimes the 
little mollusks are united by their extremities to each other, and 
two such living chains are placed side by side. These wandering 
societies have been met with forty miles long. The individuals are 
composed of a crystalline substance, tinted with red; their bodies 
are cylindrical, open at each extremity, and, like their relations, 
the pyrosoma, they are phosphorescent. These colonies of salpas 
