374 MANUAL OF ZOOLOGY SECT. 



the fin are named, respectively, the anterior, pro-pterygium 

 (bs. /), the middle, meso-pterygium (bs. 2), and the pos- 

 terior, meta-pterygium (bs.j). Of these the first is the 

 smallest, and the last the largest. The pelvic fin has only a 

 single basal cartilage, articulating with the pelvic arch, with 

 which also one or two of the fin-rays articulate directly. The 

 pelvic arch is a nearly straight bar of cartilage which runs 

 transversely across the ventral surface of the body, just in 

 front of the cloacal opening. 



The mouth leads into a very wide cavity, the pharynx 

 (Fig. 216, ph), into which opens at the sides the internal 

 apertures of the branchial clefts and of the spiracle. From 

 this runs backwards a short wide tube, the oesophagus (gut), 

 which passes behind into the stomach. The stomach is 

 a U-shaped organ, with a long left limb (cd. st) continuous 

 with the oesophagus, and a short right (pyl. st) passing 

 into the intestine. At the pylorus the point where 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 (int) is longer and very wide ; it is divisible into 

 two portions, the colon in front and the rectum behind. 

 The former is very wide and is characterised by the pres- 

 ence 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 (I. Ir, r. Ir) consisting of two elon- 

 gated lobes. A rounded sac, the gall-bladder, lies em- 

 bedded in the left lobe at its anterior end. The duct of 



