272 University of California Puhlicafions in Zoology [Vol. 18 



chains* move along with the current, seemingly quite helpless, though 

 the upper extremity is sometimes deilocted someAvhat abruptly by at- 

 tempts to escape capture. The solitary individuals, on the contrary, 

 are exceedingly active, swimming about vigorously, generally with the 

 anterior extremity uppermost ; expelling by quick and powerful jerks 

 the water which propels them by its reaction. Their motions are very 

 similar to Trachynema; they can readily change the direction of their 

 movements, and regulate them by their powerful transverse muscular 

 bands, though they lack in their motions the ease and grace of Jelly 

 Fishes. ' ' And again, on page 21 : When the individuals of a chain 

 have become separated, "the aggregate form is perfectly helpless, the 

 great thickness of the tunic preventing it from regulating its motion ; 

 while, when connected as a chain, their capacity to guide the chain in 

 any particular direction is much greater." 



Assuming, therefore, that protruding chains exist ; if the oral-atrial 

 axes of the chain salpae remain at an angle to the oral-atrial axis of 

 the solitary form after as well as before protrusion into the water 

 (see p. 244), the direction of locomotion of the solitary form will differ 

 from that of the protruding salpae. Moreover, as every salpa in the 

 left and right row respectively of the double chain, of necessity moves 

 in the same direction, the resultant force tending to propel the chain 

 at an angle to the direction of motion of the solitary form will increase 

 as the number of protruding salpae increases. The mechanical result 

 must be either to break the chain or to twist it until solitary forms and 

 aggregate forms become headed so as to move in opposite directions. 

 The situation is that of a "tug of war" in which each short chain is 

 pulled along by the solitary form while, in each long chain, the solitary 

 form is pulled along by virtue of the combined locomotive power of the 

 attached aggregate forms. 



As the number of salpae in the protruding chain increases, the 

 strain upon the chain also increases, and this must sooner or later 

 break it. Is there any clue as to how this occurs ? There is. It was stated 

 on page 244 that the stolon undergoes alternating periods of active 

 segmentation and rest so that the salpae are formed, and, according to 

 Agassiz (1866, p. 20), set free in blocks of from forty to sixty indi- 

 viduals of nearly the same size. The "intermediate piece" (Johnson, 

 1910, p. 150) connecting two blocks is composed of small, imperfect, 

 and distorted individuals. In chains of Salpa fusiforniis, which are 



* Throughout these quotations Agassiz refers not to protruding chains but to 

 chains completely isolated from the solitary salpa. 



