324 A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 



is placed at every crossing point of the ridges ; the bottom of 

 each compartment is pierced by from four to six narrow longi- 

 tudinal clefts, the branchial apertures {stigmates brancMaux, 

 Milne- Edwards), around whose edges is a fringe of numerous 

 vibratile cilia. The bridges between these clefts are hollow, and 

 receive the blood requiring aeration, brought to them on one 

 side by the ridges, which are also hollowed, and carried away, on 

 the other, after it has undergone the necessary change, to be re- 

 distributed by other ridges. The play of the cilia round the 

 edges of the clefts drives the fresh water, continually drawn into 

 the branchial sac, past the bridges. 



As we know, however, the apertures lead into a large internal 

 space {chambre thoracique et cloaque, Milne-Edwards) between 

 the respiratory sac and the second layer of the body, which 

 opens externally by the excretory or so-called anal siphon. 



Of all those parts which contribute to the complicated orga- 

 nization of the respiratory sac, the most important, that is, the 

 gill-clefts and the intervening bridges, are formed first ; for we 

 have seen the apertures appearing in the very earliest stages of 

 development, as two rounded openings on each side. It was 

 stated also that a current of blood was already established in the 

 bridges. For a long time the respiratory sac possesses only these 

 two pairs of apertures, which in the meantime elongate and be- 

 come more and more cleft-like. At last there appear two new 

 apertures on each side in the bridge between them, and soon after- 

 wards another is added behind the originally posterior cleft, so 

 that there are now altogether five openings. In this manner, 

 successively and in serial order, a multitude of openings soon 

 rapidly appear and pierce the two sides of the respiratory sac. 

 At first, a second series is formed close to and above the first, then 

 above this, a third and so more and more series, until the middle 

 line of the back is reached. In similar succession rows of aper- 

 tures appear, one below the other, between the first series and 

 the abdominal surface. Like the original openings, all these ap- 

 pear, originally, as small gaps beset with vibratile cilia, and during 

 their gradual enlargement undergo similar alterations in form, 

 becoming more and more lengthened into clefts. It is easily 

 comprehended that with the progressive increase of the respira- 

 tory sac, the number of the clefts also subsequently increases in 



