GILLS 



331 



thrown into ridges which by continued evagination rise uncovered be- 

 yond the general surface of the stratified epithelium. These evagina- 

 tions carry with them between their two walls a vascular plexus and a 

 connective-tissue support (Fig. 296). 



The blood vessels have a true capillary structure with definite en- 

 dothelial walls of their own so that the contained blood is never in actual 

 contact with the respiratory epithelium. These walls are thinnest where 



FIG. 295. Transverse section of the edge of a respiratory plate of Sycotypus. rd., one of the 

 two chitinous rods which support the edge. The blood space shows blood cells and the 

 epithelium is furnished with cilia. X 700. 



they are next to the respiratory cells to permit of the freest gas exchange. 

 They are decidedly thicker and heavier where they lie in the meshes 

 of the plexus or between the capillaries as they appear in the figure. 

 This is probably to afford support for the lamella. The epithelium as 

 a whole must be carefully studied. Where it remains on the surface 

 of the gill bar and between the lamellae it is typically stratified, the basal 

 layer proliferating freely and the proliferated layers lying in a mass that 

 reaches halfway up between the lamellae. The cells of the uppermost 

 or superficial layer of this mass are enlarged, and dense with the nucleus 



