A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 323 



the undigested remains of the food merely pass into the left 

 space and are extruded through its aperture. 



We soon see clearly that the heart pulsates more swiftly, 

 and in addition in alternately opposite directions, and that the 

 blood, which already had begun to oscillate in various parts of 

 the body, now follows a determinate, though very simple course ; 

 this consists of a dorsal and an abdominal current, united by two 

 transverse currents on each side, the one of which may be seen 

 round the anterior aperture of the body, the other within the 

 bridge formed by the wall of the respiratory sac between the two 

 respective branchial apertures. 



. As regards the test, it has already been stated that the green 

 aggregated vesicles originally deposited in it become afterwards 

 changed into its granules; this change commences only sub- 

 sequently to metamorphosis, and consists in the green struc- 

 tures gradually losing their cellularity and becoming smaller, 

 less coloured, and more angular, and so by degrees assuming 

 their ultimate form. It is only at a later period that those 

 large, rounded, thin-walled cell-spaces appear, which, as is well 

 known, occupy, closely appressed, the substance of the mantle 

 in the full-grown animal (comp. Kolliker, /. c.}. At first their 

 number is but small, but afterwards they multiply, increasing 

 in size at the same time, so that in this way the finer structure 

 of the mantle becomes more and more perfect*. 



In accordance with my plan I will now treat only of those 

 organs whose further development is either interesting in itself, 

 or has not yet been sufficiently investigated, and upon whose 

 structure the study of development is calculated to throw some 

 light. 



We may consider, first, the respiratory sac, concerning the 

 minute structure of which, in the adult animal, the following 

 points may be called to mind. It is well known that its inner 

 surface is divided into rectangular compartments by longitudinal 

 and transverse ridges, which intersect one another at right angles. 

 It is, besides, beset with numerous ciliated papillae, one of which 



* It is remarkable that the same metamorphosis of the green structures 

 occurs also in the test investing the tail, which has been left behind as an 

 empty sheath. 



21* 



