142 ZOOLOGY 



SECT. XIII 



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 (red.) 

 behind. The former is very wide and is characterised by the 

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

 brane 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. Proximally it is connected 

 with the gall-bladder, and by branch-ducts with the right and left 

 lobes of the liver. It opens into the commencement of the colon. 



The pancreas (pancr.) is a light-coloured compressed gland con- 

 sisting 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, opening 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 U-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. 773) is an antero- 

 posteriorly compressed cavity opening internally into the pharynx 

 and externally by the gill-slit. The walls of the pouches are sup- 

 ported 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 gil/s, 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 con- 

 nected branchial rays. The most anterior hemibranch is borne on 

 the posterior surface of the hyoid arch. The last gill-pouch differs 

 from the rest in having gill-plaits on its anterior wall only. On 

 the anterior wall of the spiracle is a rudimentary gill the pseudo- 

 branch or spiracular gill in the form of a few slight ridges. 



Blood system. The heart is situated in the pericardia! cavity., 



