424 



THE EVOLUTION OF MAN. 



Ftg. 152.— Transverse section throiigli the anterior part of a Lancelet. 

 (After Rolph.) The outer covering foi'ms the single cell-stratum of the outer 

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

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

 tissue pass inward from it between the muscles {M ^) and to the chorda- 

 sheath; iV, medullary tube; ch, notochord; Lli, bodj-cavity {coaJoma); A, 

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

 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) ; G, sexual 

 glands. 



