220 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



sclera which, here as elsewhere, appears as a continuation of the dural sheath of 

 the optic nerve. In the ungulates there is a retractor muscle of the ball and in 

 most mammals the superior oblique muscle of the eye passes through a loop 

 (trochlea) before it is attached to the ball. The eyes are greatly reduced in some 

 mammals, especially the moles (Talpa), the blind mole-rat (Spalax), reaching 

 the extreme in the blind marsupial, Notoryctes of Australia. In the moles the 

 reduction may consist only in a fusion of the lids, while in Notoryctes there is no 

 lens and no diflferentiation of sclera, cornea and chorioid, and the retina lacks rods 

 and cones. 



THE DIGESTIVE ORGANS 



Few articles of food, as they come to a vertebrate, are in shape to 

 be taken immediately into the organism and to be used, without modi- 

 fication, as a source of energy or as material for the construction of 

 new tissue or the repair of old. They have to be altered so that they 

 are soluble and so able to pass by osmosis into the blood-vessels 

 (proteids, carbohydrates), or they must be broken up (hydrocar- 

 bons) so as to be taken up by the absorptive vessels (lacteals) of the 

 lymphatic system. These changes in the food, which are the result 

 of the action of the secretions of the digestive glands, constitute the 

 process of digestion. The digestive tract or alimentary canal, where 

 these changes take place, also has to provide for the passage of the 

 digested food into the blood-vessels, to be carried by them to all 

 parts of the body. It is, therefore, richly supplied with blood- and 

 lymph-vessels. 



The alimentary canal, which is complete {i.e., has both mouth and 

 vent), is largely entodermal in origin, but small portions at either end 

 are derived from the ectoderm. The entodermal portion, the mesen- 

 teron, consists of the wall of the archenteron (p. lo) after the separa- 

 tion of the notochord, the mesothelium, and a few less prominent 

 structures. The ectodermal parts are a stomodeum at the cephalic 

 end and a proctodeum behind. 



In the early stages of all vertebrates the mouth is lacking, the 

 cephalic end of the archenteron abutting directly against the ecto- 

 derm of the ventral side of the head, so that an oral (pharyngeal) 

 plate is formed, consisting of both ectoderm and entoderm. Next 

 this plate is pushed inward, either as a pocket (fig. 212) or as a solid 

 plug, carrying the entoderm before it. This ingrowth constitutes the 

 stomodeimi, and the site of its ingrowth forms the mouth opening of 

 the adult. Later the oral plate breaks through, placing the stomo- 

 deum and mesenteron in communication. 



