240 CASWELL GRAVE 



by lateral blows of its tail. In the course of my study of the 

 Amaroucium tadpole, it was a great surprise to find that this 

 larva does not swim in the fashion of a vertebrate, in which a 

 constant position of the body is maintained, but that the body 

 of the tadpole while swimming is in constant and rapid rotation 

 on its long axis, clockwise as seen from behind. I have found 

 this method of locomotion also in the tadpole larva of Botryllus 

 and suspect it is characteristic of the larvae of ascidians in 

 general. On account of the suddenness with which the tadpole 

 begins to swim after one of its quiescent intervals, and the 

 rapidity with which it passes through and out of the field of 

 vision with the microscope, the rotary movement of the body 

 during locomotion is difficult to observe; however, by the addi- 

 tion of a narcotizing agent to the water, such as a solution of 

 Epsom salts, the rapidity of the swimming movements of the 

 tadpole is gradually reduced and, when moving slowly, the 

 revolutions of the body are readily noted. 



The mechanism by which the body is caused to rotate is also 

 not immediately evident. The body of the tapole is somewhat 

 compressed and, when at rest, comes to lie on one of its flattened 

 sides. For convenience in description, these flattened surfaces 

 will be referred to as the right and left sides. The side of the 

 body containing the rudiments of the oral and atrial siphons and 

 the pigmented sense organs will be referred to as dorsal, the tail 

 as posterior in position, and the end opposite to tail, containing 

 the adhesive papillae, as anterior. 



It is generally assumed that the tail fin of the ascidian tadpole 

 is expanded vertically, as in vertebrates, and in general this 

 may be the case, but the tail fin of the Amaroucium tadpole is 

 horizontal in position. 



Seeliger- noted that the tail of the larva of Clavelina lepadi- 

 formis, in consequence of its forward growth along the side of 

 the body beneath the closely fitting chorionic membrane, be- 

 comes twisted and that the nerve tube is thus turned from the 

 dorsal to the left side. According to his descriptions and figures 

 of the free-swimming tadpole, however, it follows that the tail 

 untwists when the chorion is burst, the nerve tube assuming the 



