ANATOMY AND DEVELOPMENT. 211 



One feature in connexion with the formation of the 

 atrial cavity, in which the Ascidians stand in marked 

 contrast to Amphioxus, does, however, require a special 

 explanation. 



Whereas in Amphioxus the atrial involution has the form 

 of a longitudinal groove, in the Ascidians it occurs on each 

 side, as a local inpushing of the ectoderm with a minute 

 circular orifice of imagination. 2 



The fact has already been stated above that the elonga- 

 tion of the body proper of an Ascidian embryo or larva does 

 not, in the main, take place until after the metamorphosis. 



A B 



Fig. 104. Diagrammatic transverse sections, to illustrate the mode of forma- 

 tion of the atrium in (A) an Ascidian and (B) Amphioxus. (After WiLLEY.) 



The atrial involutions occur at a time when the tail is 

 rapidly increasing its length ; the body proper, on the con- 

 trary, remaining stationary so far as increase in size is 

 concerned, and retaining at this stage approximately the 

 dimensions which it possessed when the tail first began to 

 grow out. Moreover, they occur before the appearance of 

 any gill-clefts in the wall of the branchial sac, so that in the 

 Ascidians the gill-slits never open directly to the exterior. 



In Amphioxus, on the other hand, there is no such delay 

 in the elongation of the body of the embryo, but it goes on 

 continuously till the full complement of myotomes has been 



