IXTERXAL AXATOMY. 



31 



essentially epithelial structures, are nothing else than gill- 

 filaments or gill-lamella;. 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 deri\-ative. 



Eiidoslylc. 



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

 mate structure of the phar)-nx, — the endost)-le has been 

 alread\' mentioned as a ven- 

 tral groove of the phar\nx 

 accompan^■ing 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 tenuit\' of the 

 cells, whose nuclei are often 

 placed at ilifferent le\'els, 

 gives rise to the impression 



ea 



Fig. 13. — Transverse section Iliroujh 

 endost\le of Aniphio.Kus. (Alter L.vN- 



of cells occurring in several kestkr, slightly altered.) 



i.j. Branchial artery with blood-clot, 

 layers. The four groups of .-.,-. Endostvlar coelom. ' j.<-. Skeletal plate. 



gland-cells, placed symmet- 

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

 distinguishing feature of the endostvle. The cells are 

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

 specially long cilia, which are of great importance in 

 putting in motion the cord of mucus secreted b\- the 

 glandular cells of the endostvle. Below the endost\-le, there 

 is a well-defined portion of the true bod\'-ca\'itv in which 

 the branchial artery lies. This is the t-iidostvlnr i-iv/oiu. 



