ACTIVITIES OF THE AMAROUCIUM TADPOLE 241 



dorsal, the tail fin the vertical position. Seeliger states also that 

 this larva swims in the manner of (amphibian) tadpoles. 



The slight asymmetry which exists between the right and left 

 sides of the body may account for the rotation of the body as it 

 is propelled through the water by the lashing of the tail. Viewed 

 from the dorsal side, a concave depression is seen on the left 

 located near the anterior end; moreover, the anterior tip of the 

 body, containing the middle adhesive papilla, is found to lie 

 slightly to the right of the median sagittal plane (fig. 1). These 

 asymmetrical features are the result of the pressure of the tail 

 which, during the embryonic period of development, is folded 

 forward beneath the chorionic membrane and coiled about the 

 anterior part of the body. The depression takes an oblique 

 course from below upward across the left side of the body and 

 therefore gives to it the form of a screw with a single groove 

 which tends to set up an axial rotation in the observed left to 

 right direction when the body is propelled rapidly through the 

 water. 



Considerable support for the suggestion that the screw-like 

 form of the body plays a part at least in producing rotation 

 during locomotion is found in the fact that when a tadpole sud- 

 denly stops swimming, as it frequently does during its active 

 period, the axis of the body quickly assumes a vertical position, 

 tail upward, and begins to revolve slowly from left to right as it 

 slowly sinks through the water. 



The lateral asymmetry of the body may not be the only or 

 even the chief factor in producing rotation during locomotion. 

 The direction and character of the strokes of the tail may be 

 such as to cause rotation, and my observations on the character 

 of the movements of tadpoles held captive beneath a cover-slip, 

 and on tadpoles in which a part of the tail has been amputated, 

 lead me to believe the strokes of the tail are not made directly 

 to the right or left, but that the contractions of the muscle 

 fibrillae, due to their orientation in the muscle cells, tend to 

 slightly twist as well as to bend the axis of the tail, thus produc- 

 ing a spiral thrust. The disposition of the contractile fibrillae 

 in the cortical layer of the muscle cells is shown in figure 2. As 



