TUNICATA. 29 



very widely from the ordinary conditions of their 

 relatives. They are chiefly tropical and oceanic 

 animals, swimmers in the wide and open sea, 

 visiting our coast so rarely that we can scarcely 

 reckon them as properly British animals. Hence 

 I shall but briefly notice their more obvious pecu- 

 liarities. 



These creatures are found sometimes solitary, 

 at others united in long chains composed of nume- 

 rous individuals alike in form and structure, each 

 an independent being, though constantly associated, 

 and linearly aggregated with its companions. These 

 long chains swim through the water with regular 

 serpentine movements, the result of the combined 

 reception and discharge of water by the whole 

 group. When lifted from the water, the links of 

 the chain fall asunder, the several animals of which 

 it is composed losing their power of adhesion. 



The solitary Salpse are very dissimilar from 

 these in form and structure, and are never found 

 united in chains. They were, therefore, supposed 

 to be distinct species ; but the discovery was made 

 by Chamisso, that both the one and the other 

 formed but parts of the perfect type of a single 

 species. The progeny of the Chain- Salpa is a 

 solitary Salpa, and that of the solitary Salpa is 

 a Chain-Salpa, according to the law of " Alter- 

 nation of Generations," which has already been 

 mentioned as prevailing among the Zoophyta and 

 the ACALEPHA.* 



* Professor Huxley contends that the true expression of the 

 phenomena is as follows : — The Chain-Salpa alone produces a 

 true embryo by proper generation, which becomes a solitary 

 Salpa; this, by a process of gemmation, produces a "bud," or 

 " stolon," which, under the form of a Chain-Salpa, is " nothing 

 more, homologically, than a highly individualized generative 



