STRUCTURE OF THE AMAROUCIUM TADPOLE 89 



transverse sections of the tail, but none have been observed 

 along the part of the notocord that lies within the body (figs. 2, 

 3, and 8). 



PERICARDIAL SAC AND HEART 



The pericardial sac, a thin-walled oval structure containing 

 a spacious cavity, is situated in the anterior ventral part of the 

 body beneath the anterior portion of the yolk mass. A shallow 

 but wide invagination of the dorsal wall, which extends the 

 entire length of the pericardial sac, shows the process by which 

 the heart is differentiated (figs. 1 and 2). The formation of 

 the pericardial sac of ascidians has been described as an out- 

 growth from the embryonic pharynx, but any connection that 

 may have existed at an earlier stage between these structures 

 in the Amaroucium embryo has disappeared in the fully formed 

 tadpole larva. 



THE ATRIUM 



The atrium is an extensive, U-shaped, thin-walled structure, 

 the lateral horns of which enfold the posterior portion of the 

 .pharynx (figs. 1 and 3). As has been stated in the section on the 

 pharynx, the cavities of the atrium and pharynx are connected 

 by means of three rows of gill openings. The middle portion 

 of the atrium is joined to the atrial siphon and the atrial cavity 

 is in open communication with the atrial canal. 



SUMMARY 



The points that have been added to the morphology and nat- 

 ural history of ascidian larvae as a result of this study of Amarou- 

 cium constellatum may be stated as follows: 



1. A periodicity in the escape of larvae from the parent colony 

 was observed. Large colonies can be depended upon to liber- 

 ate tadpole larvae in swarms at or near sunrise, but an occasional 

 larva escapes during other hours of the day. 



2. Multicellular, blastula-like test vesicles, about sixty in 

 number, are evaginated from the mantle during a late embry- 

 onic stage and lie isolated from the body in the substance of the 

 tunic during the entire free-swimming period of the larva. 



