262 THE EVOLUTION OF MAN. 



ribbon-like layer, by which, the intestinal canal is attached 

 to the ventral side of the notochord, is a continuation of 

 this. In addition to this, the most important parts of the 

 blood-vessel system, especially the heart, and the greater 

 arteries, also develop from this intestinal-fibrous covering. 

 In Vertebrates the intestinal canal, as a whole as well as 

 in its separate parts, is modified in various ways, although 

 its original very simple form is always the same. As 

 a rule, the intestinal canal is longer, often many times 

 longer, than the body, and therefore lies, in many convolu- 

 tions, enclosed in the cceloma, especially in the back part. 

 In higher Vertebrates it is also often divided by valves 

 into various separate parts ; the parts being distinguished 

 as the mouth, throat, oesophagus, stomach, small intestine, 

 large intestine, and rectum. All these parts arise from a very 

 simple formation, which originally (and, in the Amphioxus, 

 permanently) is a straight, cylindrical canal running from 

 front to rear below the notochord. 



As the intestinal canal, in a morphological sense, may be 

 regarded as the most important organ of the animal body, 

 it is interesting to get a clear conception of its essential 

 nature in Vertebrates, setting aside all non-essential parts. 

 In this respect, it is especially necessary to give due ; 

 weight to the fact that the intestinal canal in all Verte- 

 brates shows a very characteristic division into two parts, 

 a front half (Fig. 59, k) which serves especially for respira- 

 tion, and a hind half which serves entirely for digestion 

 (d). In all Vertebrates peculiar clefts appear, at a very 

 early period, on the right and left sides of the front divi- 

 sion of the intestinal canal ; these, the so-called gill-open- 

 ings (ks), are most closely connected to the primitive 



