SURVEY OF HUMAN INTESTINAL SYSTEM. 347 



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



(Fig. 282; Plate XI. 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. 



YI. Sixth Period -. Intestine of Primitive Fishes (p. 114). 

 Cartilaginons 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-salivarv 

 gland appears near the liver, as in Selachii. 



VII. Seventh Period .- Intestine of Dipneust a (p. 118). 

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

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

 the last section of the intestine, as in Lepidosiren. 



VHI. 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: Int est ine of Monotremes (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- 

 eavity (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 Catarhinc Apes (p. 176). 

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

 characteristic development common to Man and Catarhine Apes. 



