PYLORIC APPENDAGES OF FISHES. 431 



285, they form a circular cluster around the distal side of the 

 pylorus. Even in the longitudinally arranged caaca the principle 

 of concentration dawns ; thus the fifty pan- 289 



creatic caeca of the Pilchard communicate 

 with the duodenum by thirty orifices : but 

 the fifty attenuated terminal blind sacs in 

 the pancreas of the Lump-fish unite, reunite, 

 and discharge their secretion bv a circle 



O / One of the four bunches of 



of six orifices around the duodenal side of pyloric appendages of the 



. Whiting, isolated; showing 



the pyionc valve. In the Tunny a more their union and reunion to 



,,..,,, , n . form a single tube, ccxxxi. 



subdivided bunch ot pancreatic caeca empty 



themselves by five orifices : in the Whiting about one hundred 

 and twenty peripheral caeca progressively unite into four groups 

 or bunches, fig. 289, communicating, each by a single duct, with 

 the duodenum : in the Sword-fish, fig. 282, J, a more compact 

 gland-like mass pours its secretion into the gut by two orifices, e : 

 and, finally, in the Sturgeon and Paddle-fish, fig. 276, k, by a 

 single opening of what now becomes the short and wide duct of 

 the gland. The interposition of cellular tissue binding together 

 longer, more slender, and more ramified caeca, with a concomitant 

 increase of the vascular supply, and a common covering or 

 capsule, finally converts the accessory intestinal growths into a 

 parenchymatous conglomerate gland, as we see in the Sword-fish, 

 Sturgeon, Holocephali, and Plagiostomes ; the papilliform ter- 

 mination of the duct of such a pancreas is shown in the Selache, 

 at fig. 284, i. It sometimes exceeds the liver in weight. 



The existence of this developed form of secreting organ, over 

 and above the spiral intestinal valve, may relate to the high 

 organisation of these Cartilaginous Fishes, and to the great 

 developement of the organs of locomotion, occasioning the neces- 

 sity for rapid and complete digestion. But if we compare the 

 few existing species of heavily laden Ganoid fishes, we shall 

 again find good evidence of the compensation for a pancreas by 

 the extension of the intestinal mucous membrane within the 

 canal, the circumstances calling for a more complete developement 

 of the digestive system in the predatory Sharks and large- 

 finned Rays not being present. Thus the Polypterus, which has 

 a spiral intestinal valve, has only one short pyloric caecum, 

 fig. 279, A; whilst the Lepidosteus, which has no spiral valve, 

 has a compact group of above a hundred small caeca, which unite 

 and reunite to communicate by a few apertures with the com- 

 mencement of the duodenum. 



The inner or mucous surface of the pyloric caeca is laminated 



