A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 325 



each series, and that new series continually appear between the 

 old ones. 



Soon after the respiratory sac has become thus perforated in 

 its whole extent, we find the clefts, which previously had their 

 long diameters disposed transversely to the axis of the branchial 

 sac, arranged as in the adult animal with their long axes longi- 

 tudinal. Many of the papillae, covered with vibrating cilia, have 

 at the same time become developed, upon the inner surface of 

 the transverse and longitudinal bridges, between the clefts. The 

 blood streams rapidly and abundantly through all the bridges. 

 I have been unable to trace the development of the respiratory 

 sac any further. In its present, by 110 means complete, struc- 

 ture, however, it singularly resembles the respiratory sac of the 

 compound Ascidians. In this case we have again an example of 

 the often enunciated and repeatedly confirmed law, that the 

 higher forms of any given order of animals, at least as respects 

 particular organs, exhibit transitorily, relations of form and struc- 

 ture which occur permanently, that is, in the adult state of the 

 lower forms. 



It will be recollected, that the three apertures of the body 

 which were at first covered by the test, had opened externally 

 after the appearance of the first gill-clefts and of the heart, in 

 consequence of which the young Ascidian was enabled to take 

 in nutritive matters and the water required for respiration. 

 These apertures now become prolonged into short projecting 

 tubes, the siphons. The aperture of the respiratory siphon, 

 which from the beginning surpasses the two posterior or excre- 

 tory siphons in circumference, soon appears fringed by eight 

 lobes, as in the adult animal. In all the siphons the circular 

 fibres, by whose action they are occasionally closed, may now 

 be very readily distinguished under the test. Externally, above 

 the circular bundles, there appear also a few longitudinally fibrous 

 bundles, processes of the muscular bundles which, already inter- 

 laced, are disposed over the second layer of the body. 



The ultimate fate of the two posterior siphons is particularly 

 interesting, since, after persisting for a long time, they fuse 

 together into the single siphon present in the adult. In fact, 

 during the period in which the rapid multiplication of the 

 respiratory apertures in the branchial sac is taking place, we see 



