30 



Basic Structure of Vertebrates 



'VILLI 



PLICA CIRCULARIS 



LYMPHATIC 



^'BLOOD VESSELS 



•VILLUS 



CRYPT 



MUSCULARIS MUCOSAE 



SUBMUCOSA 



: : I'y-CIRC. MUSCLE 



LONG. MUSCLE 



SEROSA 



Fig. 22. A longitudinal section of the human jejunum, showing in cross section 

 one of the circular plicae (valvulae conniventes). (X 15.) (Courtesy, Neal and 

 Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



Fig. 23. Spiral valve of a skate, Raia. Cartilaginous fishes increase the absorbing 

 surface <>i' their intestine not by elongation, as is done by higher animals, but by a 

 spiral fold in the intestine. (After Mayer. Courtesy, Kingsley: "Comparative 

 \natoiny of Vertebrates," Philadelphia, The Blakiston Company.) 



