INTERNAL ORGANS 169 



passage, there are no salivary glands in the mouth as in the 

 higher \ertebrates. As the food leaves the mouth it passes 

 rapidly along the pharynx, the walls of which are perforated 

 by the internal branchial clefts, and enters the more or less 

 elongate gullet, from whence it is passed on to the stomach, 

 where the processes of digestion commence. Generally, a slight 

 constriction in the tube marks the boundary between the gullet 

 and stomach, but in some fishes only the change in the character 

 of the cells lining the walls serves to indicate where one begins 

 and the other ends, although the presence of gastric glands in 

 the walls of the stomach provides another clue. The stomach is 

 generally somewhat larger than the gullet or the succeeding 

 intestine, and may be U-shaped, with the concave part of the 

 U directed towards the mouth, or may take the form of a blind 

 sac with the openings for entrance and exist close together at 

 the front end. Attached near the exit of the stomach may be 

 seen in many Bony Fishes a number of blind tube-like sacs, the 

 pyloric caeca (from the Greek pyloros, a gate-keeper, and the 

 Latin caecus, blind). These may be very numerous as in the 

 Salmon (Salmo), few in number, or absent altogether, and they 

 also exhibit considerable variation in length and breadth. No 

 pyloric caeca occur in the Cat-fishes {Siluroidea) , Pikes {Esocidae) , 

 Wrasses [Labridae], Pipe-fishes [Syngnathidae) , and others; the 

 Sand Eel {Ammodytes) is said to possess a single one, the Turbot 

 {Rhombus) two, other Flat-fishes three or a few more; in the 

 Whiting (Gadus) one hundred and twenty have been counted, 

 and in the Mackerel {Scomber) no less than one hundred and 

 ninety-one. Their function is not yet properly understood, 

 although they are beHeved to secrete special juices to assist 

 digestion, and may also be concerned with the actual absorption 

 of the food into the blood. Pyloric caeca are not found in any 

 of the higher vertebrates. The walls of the stomach, although 

 provided with a strong coat of muscles, are not, as a rule, 

 particularly thick, but in certain Bony Fishes these are specially 

 modified to deal with a particular diet. In many of the lakes 

 of Ireland there is to be found a form of Trout, known locally 

 as the Gillaroo, which subsists largely on shell-fish, and as a 

 consequence has a remarkably thick-walled and muscular 

 stomach. In the Grey Mullets {Mugilidae) and in the 

 Hickory Shad {Dorosoma) of America, fishes which feed 

 largely on decomposing vegetable and organic matter mixed 

 with mud, a true gizzard like that of a fowl is developed. 

 In the Mullets the walls are so thickened that the cavity inside 



