RESPIRATION IN THE LANCELET. 431 



then in two rows, one behind the other. The foremost 

 gill-opening is the oldest. Finally, a lattice-work of fine 

 gill-openings appears on each side. 



We must call special attention to the fact that at first, 

 m the embryo of the Amphioxus, as in that of all other 

 Vertebrates, the side wall of the neck is perforated in such a 

 way by openings, that there is an open passage through the 

 latter from the external skin into the anterior intestine 

 (Fig. 158, K). The inhaled water, which is taken in to the 

 gill-intestine through the mouth, passes out directly through 

 the gill-openings. While the number of these gill-openings 

 is increasing very rapidly, over the upper row of these a 

 longitudinal fold rises, on each side, on the side-wall of the 

 body (Fig. 159, /"). The narrow body-cavity prolongs itself 

 in these longitudinal folds (Lli). Both side-folds grow 

 downward and hang as free gill-roofs. The free edges of 

 these then incline towards each other and coalesce in the 

 middle line of the ventral side, thus forming the ventral 

 seam or Raphe (Fig. 100, K). The gill-pore alone remains 

 open (Fig 15, p). Thus originates a closed gill-cavity 

 answering exactly to that of Fishes, and at the same time 

 identical with that of the Ascidians. The gill-cavity of the 

 Ascidian, the Amphioxus, the Fishes, and the larval Am- 

 phibia, are to be regarded as homologous parts. This large 

 gill-cavity, filled with water and communicating freely 

 with the surrounding water, must be distinguished from 

 the small body-cavity, tilled with lymph and without any 

 external communication. The latter, the cceloma (Figs. 

 158-160, Mi), in the adult Amphioxus is very narrow and 

 very small in size (Fig. 152, LK). When the gill-cavity 

 of the Amphioxus is complete, the respiratory water, 



