MUSEUM OF COMPARATIVE ZOOLOGY. 



331 



or five inches long are not met with, and those a foot or more in 

 length are very rare indeed, even when the water has been still for 

 some time, and are only fonnd in sheltered places, although full- 

 grown chain-zooids are very abundant. 



Huxley states of the species studied by him, which seems to be the 

 same as that found on our coast, that it " but rarely happened that 

 even two or three adhered together, and they never formed the re- 

 markable free-swimming chain of other species. Generally they were 

 found solitary, presenting only on their lateral faces traces of their 

 former adhesion" (p. 57-i). 



The chain-salpa, like the solitary form, moves by means of the 

 stream of water which is continually discharged from the atrial aper- 

 ture, and since the apertures of all the zooids of a chain are turned 

 the same way, the current produced is quite powerful, and a chain, 

 two inches long, placed in a glass bowl, with a pint of water, will 

 soon set the whole of it in rotation. As we should expect, the motion 

 of a chain is much more uniform and rapid than that of a single 

 Salpa, and they usually move in nearly straight lines, although Mr. 

 Agassiz states (Proc. Boston Soc. Nat. Hist.) that they sometimes 

 change their course to escape capture. 



Having now given a sketch of the fonnation of the chain, we will 

 go back and follow the development of the various parts more mi- 

 nutely. 



The Oater Tunic. — Little need be said of the formation of this 

 portion, — the body wall of the zooid. It is derived directly from the 

 outer wall of the tube, and its growth keeps pace with that of the 

 contained organs, which are entirely separated from it, at all the earlier 

 stages, by a distinctly visible body cavity (Fig. 30). The branchial 



Fig. 29. 



Fig. 30. 



Two stages in the development of the zonids upon the stolon : 4, constrictions which extern] in- 

 to tlic sinus chambers, and murk off the separate /""ids; 5, the ganglia ol the zooid ; '•', 

 primitive digestive organs ; s, a single ovum within the body cavity of each zooid. 



