1 90 TUNIC AT A. 



oblong, and open at both ends, tHe posterior opening being very wide, and 

 furnished with a valve so disposed that water is freely admitted, but cannot 

 again be expelled through the same channel ; so that, being forced, by the 

 contraction of the body, in powerful gushes from the opposite end, it not only 

 supplies materials for food and respiration, but impels the delicate animal 

 through the water in a backward direction. 



FIG. 190. SALPA MAXIMA. 



A very remarkable feature in the history of these creatures is 

 that many species are found swimming together adhering to each 

 other in long chains, and, what is still more strange, such aggre- 

 gated animals give birth to solitary individuals of different appear- 

 ance, which, in their turn, produce concatenated forms ; so that a 

 young Salpian does not at all resemble its mother or its daughter, 

 but is the counterpart of its grandmother or its granddaughter. 



The prodigious multitudes in which these creatures exist may 

 be gathered from the following extract : 



" Between the Cape and St. Helena, for many degrees, and in bright, breezy weather, 

 the ship passed through vast layers of sea-water so thronged with Salpse (S. mucronata} 

 as to present the consistence of jelly. These layers extended for several miles in length ; 

 what their vertical limits were it was impossible to discover. They appeared to extend 

 deep. Each of these Salpas measured about half an inch in length ; but so close was 

 their aggregation, that by a sudden plunge of an iron-rimmed tow-net, half the cubic 

 contents, from which the water had drained, consisted of nothing but one gelatinous 

 pulp. " ' ' Voyage of Sir James Ross." 



Other Ascidians are aggregated together into still more com- 

 plex assemblages. 



The Pyrosoma (Pyrosoma)* for example, is of this description. Its body 

 is made up of multitudes of Ascidians so joined together as to form a hollow 

 cylinder, open at one end, but closed at the other. The cylinder thus con- 

 structed is rowed about in the sea by the combined contractions and expan- 

 sions of all the animals composing it ; and, as it moves along, emits at night 

 a most brilliant phosphorescent light, whence the derivation of the name by 

 which it is distinguished. ' Nothing can exceed the dazzling splendour and 



* irvp, pyr,jire; au/j-a, soma, a body. 



