424 



THE EVOLUTION OF MAN. 





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

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

 skin (epidertnis, 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 (3/,) and to the chorda- 

 sheath; iV, medullary tube; ch, nofochord; Lh, body-cavity (calonia); A, 

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

 outer wall of the same; Kst, gill-rods; il/", venti-al muscles; i?, Raphe, or 

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

 glands. 



I 



