842 ARTHUR WILLEY. 



tinctly bilateral origin, in that it is formed by the fusion of 

 the two subatrial ridges. Before the actual fusion of these 

 ridges the two angles formed between the body-wall and the 

 metapleura are really the two halves of the future atrium, 

 which fuse together before being closed in. If they closed off 

 separately and fused together secondarily, we should have 

 essentially the same conditions as in the Ascidians ; always, 

 however, with the difference that in Amphioxus the two halves 

 of the atrium are in communication ventrally below the endo- 

 style, while in Ascidians they fuse together dorsally — that is, 

 on the side opposite to the endostyle. This distinction, how- 

 ever, need not be regarded as at all fundamental, when we 

 consider the extreme modification which the Ascidians have 

 undergone in comparison with Amphioxus, in correlation with 

 their sessile habit of life, and especially the absence in the 

 adult of the notochord and the reduction of the nervous 

 system. 



Fig. 5. Fig. 6. 



Figs. 5 and 6.— Diagramat.ic transverse sections to iUustrate the formation of 

 the atrium in an Ascidian (Fig. 5) and in Amphioxus (Fig. G). 



The object we have in view is to determine as far as 

 possible which organs are homologous in two related groups, 

 which, however, diverge widely from one another. As soon as 

 the homology between any two organs is established, then is 

 the time to notice the different directions in which they have 

 been modified in the respective groups; but to expect to find 

 individual organs modified in the same direction in two groups 

 in which the whole organisation has become adapted to totally 

 different conditions of life is unreasonable, and I therefore do 



