AGGREGATED SALP^E. PYROSOMA. 433 



is a hollow cylinder, from five or six to fourteen inches in length, 

 and composed entirely of minute animals of this form. These 

 are aggregated together in the following manner. A number 

 unite, like the radii of a star, so as to form a circle with a central 

 space, into which the vents of all the individuals open, their 

 mouths being at the circumference. Several of these circular 

 clusters are piled one on another, the central aperture of all of 

 them corresponding so as to form a tube ; and through one end 

 of this tube (the other being closed) a constant current of water 

 is forced out by the united ciliary action of all the individuals, 

 thus causing the mass to move through the ocean in an opposite 

 direction. The Pyrosoma is never met with but in the open 

 sea; and then it often occurs in troops composed of a great 

 number of these masses. " Nothing can be more brilliant, 

 sparkling, and lively, than the phosphoric light which these 

 animals emit. They often form long trains of fire, from the 

 manner in which the masses are disposed in cordons. But a 

 more singular phenomenon attached to this phosphorescence is, 

 that the colours vary instantaneously, passing rapidly from the 

 most lively red to the principal tints of the solar spectrum, to 

 the crimson of the morning, to orange, to greenish, to azure blue, 

 and finally to opaline yellow, when the mass is, to all appear- 

 ance, in a state of absolute repose." 



984. None of the Salpae at present known are attached to 

 each other by continuous blood-vessels, like those of the com- 

 pound Ascidiae ( 979). It is remarkable, however, that the 

 same alternation in the direction of the circulating current takes 

 place in them, as that formerly described. In fact it may be 

 regarded as characteristic of the Tunicata ; since it has been 

 observed in every species, in which the current of blood has been 

 traced. 



985. One of the most curious circumstances in the economy 

 of the compound Salpoe, is (if there be no error in the account 

 of it) that which has been described by Chamisso in regard to 

 their mode of multiplication. He states that most of the species 

 present themselves under a double form ; a race of isolated 

 animals ; and another of aggregate masses composed of nume- 



