210 THE ASCIDIANS. 



For some considerable time after the metamorphosis the 

 young Ascidian possesses two separate atrial cavities, right 

 and left, each opening to the exterior by its own atrial 

 aperture. Eventually the two cavities extend round the 

 branchial sac dorsally, so that their walls come into contact 

 in the dorsal middle line, and finally the dividing line 

 breaks down, and they become continuous one with another 

 dorsally, remaining separated ventrally, as described above. 



At the same time that the two atrial cavities grow 

 towards one another, their external apertures become in- 

 volved in the same process of growth, and, moving together, 

 finally fuse in the dorsal middle line, and so form the single 

 atrial or cloacal aperture of the adult.* 



Beyond agreeing in its ectodermal origin, there might 

 appear to be not much in common between the mode of 

 development of the atrial cavity in the Ascidians and in 

 Amphioxus. 



No morphologist would recognise a fundamental differ- 

 ence in the fact that the right and left halves of the atrial 

 cavity in Amphioxus arise by a single median involution of 

 the ectoderm, instead of from a pair of involutions, and that 

 they are from the first continuous with one another instead 

 of becoming so secondarily (Fig. 104). 



In like manner, the fact that the two halves of the atrial 

 cavity are continuous with one another ventrally in Amphi- 

 oxus and dorsally in the Ascidians, is easily brought into 

 correlation with the other differences in the organisation 

 of the two types, which have been described above, and is 

 no bar to our regarding the atrial cavity of the one as being 

 homologous with that of the other. 



* The time at which the atrial cavities fuse together varies very much in 

 different genera. In Molgula manhattensis, for instance, whose stigmata 

 develop on a similar plan to those of Ciona (see below), there is a single 

 atrial aperture at the moment of the metamorphosis. 



