A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 31? 



higher and laterally opposite to one another ; the third, which 

 springs from the middle of the anterior part of the body, is more 

 inferior. By means of these prolongations, which are seen as 

 conical processes even in the embryos (fig. 1. c, c), the larva 

 attaches itself to some appropriate place in order to go through 

 its metamorphoses. These processes, first seen by Milne- 

 Edwards, and first justly interpreted by him, and whose ex- 

 istence has been confirmed by Kolliker, have been, it would 

 seem, quite overlooked by Van Beneden. 



With regard to the portion of the test which invests the tail, 

 it may be stated here, that it terminates in a fin-like, expanded, 

 probably horizontal appendage. The true composition of this 

 appendage is, at first, the more difficult to perceive, as it can 

 rarely be got under the microscope with its whole breadth at 

 once. Therefore, it seems to me, as if the flabelliform process, 

 into which, according to Van Beneden, the test of the tail in 

 the larvae of Ascid. ampulloides (1. c. pi. 2) runs out, were a 

 similar appendage, only more strongly developed, and seen from 

 the edge. 



4. Metamorphosis and Development. The changes which the 

 tail undergoes, immediately after the attachment of the larva, 

 have been already described by Milne-Edwards in a manner, in 

 the main, in accordance with nature. According to this observer, 

 the contractile central portion or axis of the tail becomes gra- 

 dually retracted from its investment and passes at last into the 

 body of the larva, so that the investment remains behind, as an 

 empty sheath, which subsequently falls off. Milne-Edwards' 

 investigations give no account of the further fate of the retracted 

 axis. 



According to what I have seen, the loosening and retraction 

 of the axis of the tail, which, as we saw, sinks deep into the 

 body of the larva, are only the forerunners of the retrogressive 

 metamorphosis which it soon undergoes. Immediately after 

 its retraction, we find the tail still in good preservation in the 

 posterior division of the now enlarged body. Here it lies rolled 

 up spirally into a coil, which may be pressed out of the body or 

 unrolled by the use of carefully increased compression. Whilst 

 the development of the young Ascidian commences, the coil 

 breaks up first into many closely appressed lobes, and now 



