124 



BULLETIN OF THE 



branchial and cloacal apertures do not appear to be derived from the 

 atrial tunic, as they make their appearance in the positions which 

 they subsequently occupy. See Figs. 24 and 33. 



The Branchial Slits. — During the changes which have been de» 

 scribed as taking place in the lateral atria, the mid-atrium has 

 increased in size (Figs. 23, 24, and 25, i), and at last extends almost 

 from the stomach to the ganglion (v). "When seen in section (Figs. 21 

 and 33, i), it presents an irregularly triangular outline, with its base 



Fig. 26. 



Fig. 2C>, from Huxley: 6, outer tunic; !, epipharyngeal ridge; m, enclostyle ; r, heart; v, 

 stolon ; 2, central tube of stolon, which in Huxley's figure stops short, ami does not reach the 

 outer wall of the heart ; but as its connection with the latter is unmistakable, the figure lias 

 been altered accordingly. 



parallel to the posterior neural surface of the branchial sac, which 

 surface, like the corresponding one of the atrium, is flattened so 

 that they would be shown as parallel by a section at right angles 

 to the one represented in Fig. 33. The branchial and atrial tunics 

 now unite upon each side, so that the sinus (u, Fig 4) is converted 

 into a tube which communicates, at its posterior end, with the heart 

 and periviscereal sinus (Fig. 24), and at the anterior end with the 

 neural sinus. This tube is the gill or " hypopharyngeal band," and it 

 is evident that its cavity is part of the primitive body cavity, and its 

 walls are derived from or rather are parts of the branchial and atrial 

 tunics. The centres of the two regions upon the sides of the gill, 

 where these two tunics have become united, are now absorbed, so that 

 a single long and narrow branchial slit is produced upon each side of 

 the gill. The branchial cavity is thus thrown into communication 

 w'ith that of the atrium, and the upper surface of the latter now 



