424 



THE EVOLUTION OF MAN. 



Fig. 152.— Transverse section through the anterior part of 'a Lancelet. 

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

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

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

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

 sheath ; N, medullary tube ; ch, notochord ; Lh, body-cavity (ccelorna) ; A, 

 gill-cavity; L, upper wall of the latter; E 1 , inner wall of the same; E 2 , 

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

 seam formed by the coalescence of the ventral folds (gill-roofs) ; Or, sexual 

 glands. 



