INTERNAL ANATOMY. 31 



essentially epithelial structures, are nothing else than gill- 

 filaments or gill-lamellae. It appears, therefore, that we 

 are justified in supposing that the tongue-bars of Amphi- 

 oxus are the functionally active organs, of which the thymus 

 of the higher forms is a metamorphosed derivative. 



Endostyle. 



Returning, then, to the consideration of the more inti- 

 mate structure of the pharynx, --the endostyle has, been 

 already mentioned as a ven- 

 tral groove of the pharynx 

 accompanying the latter 

 throughout its whole length. 

 A transverse section of it 

 alone is shown in Fig. 13. 

 It is composed of very high 

 columnar cells arranged 

 throughout in one layer, al- 

 though the tenuity of the 

 cells, whose nuclei are often 

 placed at different levels, 



p-ives rise to the imnression Fig> '3- -Transverse section through 

 S 1V endostyle of Amphioxus. (After LAN- 



of cells occurring in several KESTER. slightly altered.) 



e.a. Branchial artery with blood-clot, 

 layers. The four groups of e .c. Endostylar ccelom. sk. Skeletal plate. 



gland-cells, placed symmet- 

 rically two on either side of the median line, are the 

 distinguishing feature of the endostyle. The cells are 

 all ciliated, but those in the middle line bear a bunch of 

 specially long cilia, which are of great importance in 

 putting in motion the cord of mucus secreted by the 

 glandular cells of the endostyle. Below the endostyle, there 

 is a well-defined portion of the true body-cavity in which 

 the branchial artery lies. This is the endostylar cozlom. 



