342 



Vertebrata. 



the body. In correlation with the importance of the small intestine 

 as an organ of absorption there are various arrangements for increas- 

 ing its inner surface, folds arranged in a network, or papilla (villi), 

 the latter chiefly occurring in the Mammalia. At the anterior end of 

 the small intestine opens the duct (or ducts) of the liver, a very 

 large, often lobed gland, consisting of very numerous lobules; the 

 duct (bile duct) is usually provided with a saccular expansion, the 

 gall bladder, which forms a reservoir for the hepatic secretion, 

 the bile. Close to the opening of the bile duct into the gut is the orifice 

 of another large gland, the pancreas, which, like the liver, is 



usually extensive; it is absent 

 ^ S from some Msh. Besides 



these larger glands external 

 to the intestine, there are 

 frequently numberless small 

 tubular or racemose glands 

 lying in its wall {e.g., in 

 the Mammalia) . The terminal 

 portion of the alimentary 

 canal is the rectum, which 

 is wider than the small intes- 

 tine. In most Vertebrata it 

 is short, and is then straight 

 also; it only attains a greater 

 length in the Mammalia; 

 here it is termed the large 

 intestine, the term rec- 

 tum being reserved for the termination only. In mauy Vertebrata 

 the hinder end of the rectum acts as a cloaca, the urinary 

 and genital ducts opening into it. At its anterior end, at its junction 

 with the small intestine, the rectum (or large intestine) is often 

 provided in Reptiles and Mammals with one, in Birds with two, c se c a 

 of varying length.. The anus is situated ventrally at the base of 

 the tail, it is either round or a longitudinal or transverse slit. 



The alimentaiy canal in the embryo is for some time a straight tube ranning 

 tkrough the body-cavity along its dorsal surface, invested with a thin connective 

 tissue membrane, the peritoneum, which covers all the organs in the body- 

 cavity. Later it leaves the body- wall and sinks deeper into the cavity, drawing 

 its pei-itoneal covering with it, so that in the adult it is suspended by a large fold 

 of the peritoneum (Pig. 282). Where they do not sui-round the alimentary canal 

 the two layers of this fold lie close together, and form the mesentery, which 

 appears as a thin connective tissue lamina between the upper body-wall and the 

 digestive tract. Similar mesenteries may also be formed for other organs in the 

 body-cavity. 



The respiratory organs of Vertebrata are sometimes 

 gills, sometimes lungs, the latter occurring in almost all 

 (with the exception of Amphioxus, the Cyclostomes, the Selachians, 



Fig. 282. Diagram to explain the structure of 

 the mesentery; transverse section of the 

 body. A earlier, B later stage, d gut, s peri- 

 toneum (thick line), s' the portion of this sur- 

 rounding the gut ; m mesentery, formed of two 

 layers ; r spinal cord. — Orig. 



