SURVEY OF HUMAN INTESTINAL SYSTEM. 347 



IV. Fourth Period : Intestine of Skull-less Animals (Acrania) 

 (Fig. 282; Plate XL Fig. 15). 

 The gill-streaks appear between the gill-openings of the respiratory 

 intestine ; a liver blind-sac grows from the stomach-pouch of the digestive 

 intestine ; as in the Amphioxus. 



V. Fifth Period : Intestine of Cyclostoma (Plate XI. Fig. 16). 

 The thyroid gland develops from the ciliated groove on the base of 

 the gills (hypobranchial groove). A compact liver-gland develops from 

 the liver blind-sac. 



VI. Sixth Period : Intestine of Primitive Fishes (p. 114). 

 Cartilaginous gill-arches appear between the gill-openings. The fore- 

 most of these form the lip-cartilages and the jaw-skeleton (upper and lower 

 jaw). The swimming-bladder grows from the pharynx. The ventral-salivary 

 gland appears near the liver, as in Selachii. 



VII. Seventh Period : Intestine of Ptipmeusta (p. 118). 



The swimming-bladder modifies into the lungs. The mouth-cavity 

 becomes connected with the nose-cavity. The urinary bladder grows from 

 the last section of the intestine, as in Lepidosiren. 



VIII. Eighth Period: Intestine of Amphibia (p. 126). 



The gill-openings close. The gills are lost. The larynx originates from 

 the upper end of the trachea. 



IX. Ninth Period: Intestine of Monotr ernes (p. 145). 

 The primitive mouth and nasal cavity is separated by the horizontal 

 palate-roof into the lower mouth-cavity (food passage) and the upper nose- 

 cavity (air passage); as in all Amnion Animals. 



X. Tenth Period: Intestine of Marsupials (p. 149). 



The existing cloaca is separated by a partition wall into an anterior 

 urinary-sexual aperture and a posterior anal aperture. 



XI. Eleventh Period : Intestine of Catarhine Apes (p. 176). 

 All parts of the intestine, and especially the teeth-apparatus, acquire the 

 characteristic development common to Man and Catarhine Apes. 



