768 THE INTESTINE. 



there consists of a fold in the wall of the intestine, giving to the 

 lumen of this canal a semilunar form in section, and taking a 

 half spiral. 



In Elasmobranchii a similar fold to that in Ammocoetes first 

 makes its appearance in the embryo. This fold is from the 

 first not quite straight, but winds in a long spiral round the 

 intestine. In the course of development it becomes converted 

 into a strong ridge projecting into the lumen of the intestine 

 (fig. 388, /). The spiral it makes becomes much closer, and it 

 thus acquires the form of the adult spiral valve. A spiral valve 

 is also found in Chimaera and Ganoids. No rudiment of such 

 an organ is found in the Teleostei, the Amphibia, or the higher 

 Vertebrata. 



The presence of this peculiar organ appears to be a very 

 primitive Vertebrate character. The intestine of Ascidians 

 exhibits exactly the same peculiarity as that of Ammoccetes, 

 and we may probably conclude from embryology that the 

 ancestral Chordata were provided with a straight intestine 

 having a fold projecting into its lumen, to increase the area of 

 the intestinal epithelium. 



In all forms in which there is not a spiral valve, with the 

 exception of a few Teleostei, the intestine becomes considerably 

 longer than the cavity which contains it, and therefore neces- 

 sarily more or less convoluted. 



The posterior part usually becomes considerably enlarged to 

 form the rectum or in Mammalia the large intestine. 



In Elasmobranchii there is a peculiar gland opening into the 

 dorsal side of the rectum, and in many other forms there is a 

 caecum at the commencement of the rectum or of the large 

 intestine. 



In Teleostei, the Sturgeon and Lepidosteus there opens into 

 the front end of the intestine a number of caecal pouches known 

 as the pancreatic caeca. In the adult Sturgeon these pouches 

 unite to form a compact gland, but in the embryo they arise as 

 a series of isolated outgrowths of the duodenum. 



Connected with the anterior portion of the middle region of 

 the alimentary canal, which may be called the duodenum, are 

 two very important and constant glandular organs, the liver and 

 the pancreas. 



