i8 ZOOTOMY. 



54. The branchial filaments, sub-parallel ridges of 

 the mucous membrane, lining the gill-sacs, and encroaching 

 largely upon the cavity of the sacs. 



The whole set of filaraen's on each side of each sac constitutes a 

 demibranch or half-gill : the septum between any two sacs, together 

 with the two contiguous demibranchs, answers to a single gill of one 

 of the higher fishes. (See p. 46, § 84, and p. 103, § 89.) 



55. The internal branchial apertures (Figs. 4 and 

 6, i.a), on the inner walls of the gill-sacs, leading into the 

 respiratory tube presently to be seen (§ 58). 



56. The peribranchial sinuses {Fig. 6, p.br], considerable 

 spaces included between the gill-sacs themselves and the partitions 

 which separate them. 



X. Remove the gill-sacs, cutting them away close round 

 the internal branchial apertures, but taking great 

 care not to injure the respiratory tube (§ 58), 

 gullet (§ 59), &c. : also dissect away enough of 

 the dorsal body muscles to allow of the structures 

 mentioned being brought clearly into view. Ob- 

 serve the following : — 



57. The retractor muscles of the tongue (Fig. 4, 

 r.m.t), surrounding the lingual cartilage (§ 14, Figs, i 

 and 4, Ig), and forming a prominent cylindrical fleshy mass, 

 in the middle ventral line, separating the gill-sacs of opposite 

 sides, and extending back as far as the pericardium. 



58. The delicate membranous respiratory tube (Figs. 

 4 and 6, ;-./), lying almost immediately above the retractor 

 of the tongue ' : the seven internal branchial apertures of 

 the left side are seen perforating its lateral wall. 



59. The oesophagus (Figs. 4 and 6, ces), lying imme- 

 diately above the respiratory tube, and becoming continuous 



' The ventral aorta which is between the two will be mentioned 

 subsequently (§ 72). 



