SURVEY OF IIl'.MAN INTESTINAL SYSTEM. 347 



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



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



The gill-streaks appear between the gill-openinps of the respratory 

 iutestine ; 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 bliud-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 j:iw-skek ton (upper and lower 

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

 gland appears near the liver, as in Selachii. 



VII. Sei-mth 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 L,epidosiren. 



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 ofMonotremes (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 Aniniou Animals. 



X. Tenth Period: Intestine ofMarsv^'al* (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: Intrxtine of Catarhine Aprs (p. 176). 

 AH parts of the intestine, and especially the teeth-apparat-i.-., acqniro the 

 charuoteribtic development common to Alan and Catarhiue Apes. 



