AGGREGATED ASCIDIANS. SALPA. 



433 



along in an upright position, apparently without any power of 

 controlling its motions. 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 bril- 

 liant, 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." 



ORDER II. SALP^E. 



1066. The Salpce. are animals of greater delicacy of organisa- 

 tion than the greater part of the Ascidians. They are usually 

 so transparent, that the interior structure may be examined 



p 



FIG. 703 SAT.PA ; a, anterior orifice ; p, posterior or anal orifice ; /, nucleus, Inclosing the vis. 

 cera ; e, heart ; br. branchia ; nt, muscular bands ; n. nervous ganglion. * 



through their tunic with little difficulty. They consequently 

 seldom possess any decided colour, except when viewed in a 



