SURVEY OF HUMAN INTESTINAL SYSTEM. 



347 



IV. Fourth Period : Intestine of Skull-less Animala {Acra/nta) 

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

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

 intestine ; a liver blind.sao grows from the stomach-pouoh of the digestire 

 intestine ; as in the Amphioxru. 



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

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

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

 the liver blind-sac. 



VI. 8i«Bth 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 Dipneusta (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. 



Vni. 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 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- 

 cavity (air passage) ; as in all Amnion Animals 



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

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

 urinary-sexual aperture and a posterior anal aperture. 



XI. Eleventh Period : Intestine of Cata/rhine Apes (p. 176). 

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

 ohaxaoteristio development common to Man and Gatarhine Apes. 



