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 (corium), the 

 inner tissue of which is thickened below (TJ) ; 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 (cceloma) ; A, 

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

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

 seam formed by the coalescence of the ventral folds (gill-roofs) ; 0, sexual 

 glands. 



