320 A. KROHN ON THE DEVELOPMENT OP THE ASCIDIANS. 



into a regular current ; at the same time the, originally, quite 

 simple main trunks and brancheshave become double, whilst their 

 globularly enlarged, terminal twigs, directed towards the peri- 

 phery of the mantle, are still simple tubes. In the larger vessels, 

 which accompany one another and branch out in common, just 

 as in the adult animal, the blood now streams in two opposite 

 directions, in the one vessel towards the periphery, in the other 

 towards the heart. Shortly before the division of the last branches 

 into the terminal ramuscules, we see these two streams pass into 

 one another by a loop ; on the other hand, in the terminal 

 branches no continuous current of the blood is as yet to be ob- 

 served; the blood-corpuscles indeed penetrate into them, but they 

 frequently stagnate and sometimes become excessively accu- 

 mulated. They are only occasionally seen to enter the current 

 and return in one or other of the streams we have mentioned. 

 All this continues until the doubling has extended as far as the 

 terminal twigs. The cause of the two opposite currents is 

 readily discovered, inasmuch as observation shows that each of 

 the double vessels opens into opposite ends of the heart. Since, 

 furthermore, the heart, even soon after its appearance, pulsates 

 periodically in two opposite directions, at every change, we see 

 the current of blood in each vessel reversed. 



Van Beneden has described processes very analogous to the 

 rudiments of the vessels of the test, growing forth in the Ascidia 

 ampulloides, soon after the retraction of the tail ; after a short 

 existence, however, they disappear. If these last processes are 

 identical with the vessels of the test of the developing Phallusiw^ 

 the latter statement contradicts the observations communicated 

 above. Ascidia ampulloides is very probably a Cynthia. The 

 cartilaginous test, the plaited respiratory sac, and the double 

 reproductive organs, arranged exactly as in the Cynthia, speak 

 for this view. In the mantle of the CynthicR, so far as I know, 

 no vessels have as yet been demonstrated. If they be absent, 

 then the processes indicated by Van Beneden must have some 

 other import ; if they be present, the statement that the pro- 

 cesses soon disappear is probably erroneous*. 



* The budding vessels of the test will not easily be confounded with the 

 puzzling diverticula of the test, which, Proteus-like, sometimes protrude, some- 

 times are again retracted and disappear wholly, observed by Milne-Edwards 



