424 



THE EVOLUTION OF MAN. 



FIG. 152. Transverse section through the anterior part of a Lancelet. 

 (After Eolph.) The outer covering forms the single cell-stratum of the outer 

 skin (epidermis, E). Below this lies the thin leather skin (cerium), the 

 inner tissue of which is thickened below (U) ; partition walls of connective 

 tissue pass inward from it between the muscles (M J and to the chorda- 

 sheath ; N, medullary tube ; cJi, notochord ; Lh, body-cavity (cceloma) ; A, 

 gill-cavity; L, upper wall of the latter; E lt inner wall of the same; E 2 , 

 outer wall of the same ; Kst, gill-rods ; M, ventral muscles ; R, Raphe, or 

 seam formed by the coalescence of the ventral folds (gill-roofs) ; G, eexual 

 glands. 



