INTERNAL ANATOMY. 



27 



Structure of Pharynx. 



We have already had occasion to mention the fact that 

 the wall of the pharynx on each side is perforated by a 

 great number of vertically elongated slit-like apertures — 

 the gill-clefts. In the middle region of the pharynx the 

 gill-slits stretch almost from the roof to the base of the 

 pharynx, but in front and behind they gradually become 

 much lower in vertical height (Fig. 10), In the fully 



Fig. 10. — Anterior poriion of right wall of pharynx, to show arrangement of 

 skeletal rods. (After J. MUller.) 



e. Endostyle. e.c. Endostylar coslom. p.lK Skeletal rod of primary gill-bar. 

 t.b. Skeletal rod of tongue-bar. sy. Cross-bars or synapticula. 



N.B. — A simple gill-slit undivided by a tongue-bar should liave been inserted 

 in the figure in front of the first double slit. J. Miiller failed to observe this. 



expanded condition the gill-slits are nearly vertical, as in 

 Fig. 10, but by the contraction of the transverse muscles, 

 which lie in the floor of the atrium, they are often found 

 to be directed very obliquely backwards, and this is the 

 condition in which they almost invariably occur in pre- 

 served specimens. That is the reason why so many of 

 the bars are involved in a single transverse section. (Cf. 

 Fig. 2.) On account of the prodigious extent to which 



