514 Mr. A. Willey. On the Post-Embryonic [May 19, 



Cb.) The club- shaped g land of 1 m , • , <. A 



A t . r — 1 he intestine of Ascidians. 



Amphioxus. J 



(c.) Gill-slits of Amphioxus, unrepresented in Ascidians. 

 (cZ.) Atrial cavity of Amphioxus, unrepresented in Ascidians. 



Of the above propositions, the last may be true ;but I shall proceed 

 to show that the others are untenable. 



Fixation of the Larva of Ciona. 



Just before the larva fixes itself, a narrow space can be discerned 

 between the anterior end of the endoderm and the tract of ectoderm 

 which bears the adhering papillae ; and in it lies a compact group of 

 mesoderm cells. 



Shortly after fixation this space swells up prodigiously, and then 

 contains loose scattered mesoderm, cells. I shall call this the pro- 

 boscis-cavity. At its base (i.e., where it joins on to the body) lies the 

 endostyle dorso-ventrally. The primary position of the endostyle is 

 extremely important. It behaves exactly as it does in the larva of 

 Amphioxus, in that it occupies at first the most anterior region of the 

 alimentary canal, and lies at right angles to the position which it 

 assumes later. 



In fact, the trunk of the young fixed Ascidian undergoes an 

 actual rotation through an angle of 90° as the result of which the 

 mouth, which was at first dorsal, becomes terminal, and the endostyle 

 takes up its definite longitudinal position at the base of the branchial 

 sac. 



In comparing the accompanying figures (1 and 2) the attention 

 should, first of all, be concentrated on the endostyle, and then the 

 structures which precede and follow it in both cases should be taken 

 into consideration. When that is done, I think the inadmissibility of 

 van Beneden's and Julin's view of the homology of the first pair of 

 gill-slits (primary branchial canals) of Ascidiars with the proboscis- 

 cavity and praeoral pit of Amphioxus (anterior intestinal diverticula) 

 will at once become evident. 



In Clavelina the behaviour of the proboscis-cavity is essentially 

 the same as in Ciona. 



Origin of the Gill-slits in Ciona and Clavelina. 



The post-larval appearance of the gill-slits of the simple Ascidians 

 has been studied to a certain extent only by P. J. van Beneden, 

 Krohn, KupfFer, and Ed. van Beneden and Julin ; and most 

 thoroughly by the last two authors. The first three observed young 

 Ascidians with two branchial apertures on each side. Van Beneden 

 and Julin (" Rech. sur le Developpement postembryonnaire d'une 



