252 TUNICATA. 



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. 



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 aggregated animals 

 give birth to solitary individuals of different ap- 

 pearance, 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 (8. mueronata) as to present the con- 

 sistence 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 SalpsD 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 Boss. 



Other Ascidians are aggregated together into still 

 more complex 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 constructed is rowed about in the 

 sea by the combined contractions and expansions of 

 all the animals composing it ; and as it moves along, 



* 7ri>f>, pyr, fire ; a-upa, soma, a lody. 



