XIII 



PHYLUM CHORDATA 



149 



the stomach passes into the intestine is a slight constriction, 

 followed by a thickening. The intestine consists of two parts small 

 intestine or duodenum, and large intestine. The former is very short, 

 only an inch or two in length. The latter is longer and very wide ; 

 it is divisible into two portions the colon (col.) in front and the 

 rectum (rect.) behind. The former is very wide and is characterised 

 by the presence in its interior of a spiral valve, a fold of the mucous 

 membrane which runs spirally round its interior, and both retards 

 the too rapid passage of the food and affords a more extensive 

 surface for absorption. The rectum differs from the colon in being 

 narrower and in the absence of the spiral valve ; it opens behind into 

 the cloaca. 



There is a large liver (liv.) consisting of two elongated lobes. A 

 rounded sac the gall-bladder (g. bl.) lies embedded in the left 

 lobe at its anterior end. The duct of the liver the bile-duct (b.dct.) 

 runs from the liver to the intestine. Proxinially it is connected 

 with the gall-bladder, and by branch-ducts 

 with the right and left lobes of the liver. It 

 opens near the commencement of the colon. 



The pancreas (pancr.) is a light-coloured 

 compressed gland consisting of two main 

 lobes with a broad connecting isthmus, lying 

 in the angle between the right-hand limb of 

 the stomach and the small intestine. Its 

 duct enters the wall of the small intestine 

 and runs in it for about half an inch, open- 

 ing eventually at the point where the small 

 intestine passes into the colon. 



Connected with the rectum on its dorsal 

 aspect is an oval gland the rectal gland 

 (rect. gl.) about three-quarters of an inch in length. 



The spleen (spl.) is a dark-red or purple body attached to the con- 

 vexity of the (J -shaped stomach and sending a narrow lobe along 

 the right-hand limb. 



The organs of respiration in the Dog-fish are the gills, situated 

 in the five gill-pouches. Each gill-pouch (Fig. 828) is an antero- 

 posteriorly compressed cavity opening internally into the pharynx 

 and externally by the corresponding gill-slit. The walls of the 

 pouches are supported by the branchial and hyoid arches with their 

 rays, the first pouch being situated between the hyoid and first 

 branchial arches, the last between the fourth and fifth branchial 

 arches. On the anterior and posterior walls of the pouches are the 

 gills, each hemibranch consisting of a series of close-set parallel 

 folds or plaits of highly vascular mucous membrane. Separating 

 adjoining gill-pouches, and supporting the gills, are a series of broad 

 interbranchial septa, each containing the corresponding branchial 

 arch with its connected branchial rays. The most anterior hemi- 



FIG. 828. Hemiscyllium. 

 Branchial sac exposed from 

 the outside. 



