PHARYNX. 



77 



having the same Vertical extent as the gill-slit and placed close 

 to the pharyngeal wall (Fig. 61). This branchial pouch opens 

 to the exterior by a small aperture the branchial pore. The 

 whole branchial passage is lined by endoderm and is developed 

 as a pharyngeal diverticulum. In the forms just mentioned, the 

 U-shaped gill-slits open directly to the exterior and there are no 

 branchial pouches or pores distinct from them. This is due to 

 the fact that the original pharyngeal outgrowth remains short and 

 its external opening becomes as wide as the internal. The gill- 

 apertures are added to throughout life and arise as small round 

 perforations at the hind end of the pharynx. The tongue is de- 

 veloped as a down-growth of the dorsal 

 wall as in Amphioxus, but hangs freely, 

 not as in that animal becoming con- 

 nected with the ventral margin (Figs. 

 60, 62). It receives a prolongation of 

 the trunk coelom (p. 92). Synapticula 

 (Fig. 62) are present in some species, 

 traversing the two limbs of the U-shaped 

 aperture. They are connected to the 

 tongue bar, and vary considerably in 

 number in the different species ;. in Bal. 

 carnosus there may be as many as 30 

 on each side of a tongue bar ; 10 to 

 1 1 seems however to be the usual num- 

 ber. They are absent in the Harri- 

 maniidae and in Glandiceps. In most 

 forms the gill - slits are not quite 



straight, being slightly bowed with the concavity directed 

 forward. 



The successive branchial pouches are placed close together, 

 and their opposed walls constitute the branchial septa. The 

 first of them always receives the opening of the collar pore 

 (see below), and in Bal. carnosus the first branchial pore opens 

 into the hind end of the medullary canal. The gill- apertures are 

 very numerous. The number varies of course with the growth 

 of the animal, and appears to be highly variable in the different 

 species. It varies from 10 or 11 pairs, the number found in 

 Dolichoglossus sulcatus, to 700. the number recorded for Bal. 

 aurantiacus. 



FIG. 62. Diagram of two gill- 

 slits of Ptychodera viewed from 

 the inner side (from MacBride). 

 1 gill-slit ; 2 tongue-bar ; 3 

 synapticulum ; 4 septal or 

 primary bar ; 5 skeletal rod of 

 primary bar forking ventrally ; 

 6 skeletal rod of tongue-bar, 

 double. 



