212 THE ASCIDIANS. 



formed. The post-anal portion of the body, which we sup- 

 pose to be the homologue of the tail of the Ascidian tad- 

 pole, does not appear until a somewhat late period in the 

 development. There is very little of it present in the larva 

 with three gill-slits (Fig. 73). 



The reason of this, as explained above, is that the post- 

 anal section of the trunk is of only minor functional sig- 

 nificance in Amphioxus, but is all-important to the 

 Ascidian larva, and consequently, as is the case with 

 many other structures of great functional importance in 

 the various groups of the animal kingdom, it exhibits a 

 precocious development. 



Not only, therefore, has the elongation of the body of 

 Amphioxus already taken place before the occurrence of 

 the atrial involution, but the primary gill-slits have also 

 broken through the wall of the pharynx, and open freely to 

 the exterior before the atrium begins to be closed in. 



In Amphioxus, then, the atrial involution has been drawn 

 out into the form of a longitudinal groove because it 

 occurs subsequently to the elongation of the body and 

 the perforation of the gill-slits. 



In the Ascidian embryo the (paired) atrial involution 

 has the form of a simple pit with a circular margin, be- 

 cause it arises before the elongation of the body proper 

 of the embryo and before the perforation of the gill-clefts, 

 so that no influence has been at work to draw it out into 

 the form of a groove. 



We see, therefore, that a great many of the differences 

 between the Ascidian tadpole and the larva of Amphi- 

 oxus can be explained sufficiently to allow of their being 

 brought into genetic relation with one another, by consid- 

 ering the relative time at which corresponding develop- 

 mental processes take place in the two cases. 



