322 A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 



be distinguished in it, the oesophagus opening into the respi- 

 ratory sac, the stomach, and the intestine. 



Soon after this there appear in the wall of the respiratory sac 

 the first respiratory or gill-clefts (stigmates branchiaux, M. Edw.), 

 in the form of four round apertures, provided with vibrating 

 cilia. Their distribution is perfectly symmetrical, each pair 

 lying upon the two opposite sides of the respiratory sac, one 

 close behind the other and below their respective excretory 

 apertures. Van Beneden (1. c. p. 44. pi. 3. fig. ll/".) has ob- 

 served these apertures and discerned their true import. 



The heart appears to be formed last of all. It represents at 

 first, a very short sac lying on the right side, near the stomach, 

 or rather near the abdominal furrow, distinguishable by its un- 

 dulating but, at present, very slow movement. It is distended 

 with blood-corpuscles, which are driven alternately hither and 

 thither. 



Before long the test gives way, over the three apertures of the 

 body, whose margins have, in the meantime, become notched, and 

 thus the young Ascidian enters into relation with the external 

 world, since, from this time forth, it is enabled to take in nutritive 

 matter and the water necessary for respiration. Before this, 

 however, the second layer of the body on each side, in the entire 

 neighbourhood occupied by the two apertures of the gills, has 

 become detached from the respiratory sac, which elsewhere lies 

 in close apposition with it, in the form of an externally convex 

 roof. This roof therefore includes a cavity, which communicates 

 on the one hand, by means of the respiratory apertures with the 

 branchial sac, on the other by means of the corresponding ex- 

 cretory apertures, which are seated upon the summit of the roof, 

 with the exterior ; the terminal portion of the intestine, which 

 has in the meanwhile become longer, curves at this time round 

 the bottom of the respiratory sac upwards, and opens in the end, 

 by means of the anus into the left space above mentioned*. In 

 this way it becomes possible that the water drawn in through 

 the anterior aperture of the body into the respiratory sac, should 

 pass through the branchial apertures into the two spaces, and 

 become emptied out again through the excretory apertures, whilst 



* We shall subsequently recur once more to these two spaces. 



