STRUCTURE OF THE AMAROUCIUM TADPOLE 77 



nature of the extruded material is shown by the fact that the 

 tadpole adheres to any foreign body against which it chances 

 to swim, and the most violent movements often fail to release 

 it from such an attachment. 



TEST VESICLES 



In the fully developed tadpole a large number of blastula-like 

 vesicles occupy a considerable part of the space in the anterior 

 median region of the tunic. They have no organic connection 

 with the body, but lie midway between the mantle and the ex- 

 ternal surface of the tunic. They are separated by the stalks 

 of the adhesive papillae into four unequal groups (fig. 1). The 

 part of the wall of each test vesicle turned toward the surface 

 of the tunic is composed of cells much larger than those on 

 the side facing the body (fig. 9). The test vesicles maintain 

 this position and orientation during the entire free-swimming 

 period of the tadpole. The function and history of these bodies 

 formed the subject of a paper (Grave, '20b) prepared for the 

 program of the seventeenth annual meeting of the American 

 Society of Zoologists, an abstract of which has been published 

 in the Proceedings of the meeting. 



Each test vesicle takes its origin from the mantle wall in the 

 form of a hollow, club-shaped outgrowth or evagination during 

 the late embryonic period. Four clusters of these club-shaped 

 bodies, attached to four median elevations of the body wall, 

 may be seen in immature tadpoles. The dorsal and ventral 

 groups project from keel-like ridges, while the anterior groups 

 are attached to conical papillae situated midway between the 

 bases of the stalks of the adhesive papillae, and each has the 

 appearance of a bouquet or rosette (fig. B). Each club-shaped 

 outgrowth ultimately becomes coverted into a test vesicle, 

 first by the appearance of a constriction near its point of attach- 

 ment, then at the point of the constriction it separates from the 

 body as a pear-shaped structure which gradually assumes a 

 spherical form and migrates to a position in a zone midway be- 

 tween the body wall and the external surface of the tunic. The 

 test vesicles of Amaroucium probably correspond to the 'bladder 



