A. KROHN ON THE DEVELOPMENT OP THE ASCIDIANS. 319 



are always found enlarged in the mode described. In young 

 developing Ascidians which have fixed themselves normally, that 

 is, by their whole under surface, we see subsequently, when the 

 vessels of the test have branched out extensively, that all the 

 branches and twigs extend perpendicularly and radially, on all 

 sides, towards the circumference of the mantle. In those indivi- 

 duals, on the other hand, which have been unable to find any, or 

 no convenient, place for fixing themselves, while still larvae, but 

 whose development nevertheless goes on*, the original vessels of 

 the test, namely the three above-mentioned processes, as well 

 as their subsequent ramifications, take the most various direc- 

 tions, curving downwards for the most part. It would seem, 

 therefore, as though a speedy termination were put to the further 

 ramification by the above-mentioned unfavourable circum- 

 stances ; such individuals probably die prematurely. 



It is, however, so much the easier to mistake the true nature 

 of the vessels of the test, during the early stages, since even when 

 the heart has appeared and the circulation has become established, 

 we see not the slightest trace of any current of blood in them. 

 In addition, the mantle is so transparent that its contours are 

 easily overlooked, and the idea may readily arise, that the much 

 more sharply defined vessels extend beyond its boundaries, 

 whereas they are invested by it. It is therefore difficult, at first, 

 to avoid considering the vessels of the test as stolons or runners, 

 by the aid of which the young Ascidian seeks to fix itself more 

 and more strongly to its residence. This view has the greater plau- 

 sibility because msiny A.scidians{Cynthiapapillata for example) do, 

 in fact, attach themselves to foreign bodies by means of branched 

 stolons ; however, the establishment of the circulation in these 

 vessels of the test at once releases us from this error. At first 

 one sees nothing but a column of blood containing merely a few 

 granules, irregularly oscillating. Afterwards, when the ramifi- 

 cations have extended, the blood is more rich in granules and 

 the circulation goes on more rapidly ; the oscillation is converted 



* A yet more striking example how little the course of development allows 

 itself to be arrested, is presented by certain larva? which have been unable to 

 burst their envelopes, but whose metamorphosis has nevertheless commenced, 

 as we see by the coiled tail and the commencing formation of the vessels of the 

 test. 



