50 



ME. E. H. BUENE ON THE AOETIC-ARCH 



all the others on the same side in magnitude. The right, on the 

 contrary, is smaller than all preceding it on its side. The left 

 fourth branchial artery leaves the fourth gill-arch to pass to the 

 ventral wall of the dorsal respiratory sac, on which it passes to the 

 hinder end of the sac, giving off alternating side-twigs. On the 

 right, the artery passing to the respiratory sac is not a pro- 

 longation of the fourth, but of the first branchial artery, and 

 runs not on the ventral, but on the dorsal wall of the sac." 

 This statement of Hyrtl's is endorsed by Hubrecht*, who dis- 

 sected one of these fishes at the request of Day when the latter 

 was working at the physiology of this apparatus. 



Brancbial region (nat. size) of Saccobranchus fossilis, showing the arrangement 

 of the branchial arteries, seen from the ventral aspect. — i, ii, iii, iv, branchial 

 arteries ; r.s., respiratory sacs. 



In a specimen of Saccobranchus fossilis in the Museum of the 



Eoyal College of Surgeons (No. 1061 Gr), which I dissected last 



year, the branchial arteries do not answer to this description, 



fur here the arteries are quite symmetrical on either side. The 



* Day, Joiiru, Linn. iSoc. (Zool.) toI. xiii. p. 198. 



