258 



PISCES 



Islands of Langerhans in the mesentery furnish insulin. (See p. 

 156.) 



Elasmobranchs have two structures, the paired suprarenals and 

 interrenals. The suprarenals are segmentally arranged bodies 

 situated in pairs on the segmented arteries given off from the dorsal 

 aorta. The first pair are called axillary hearts. The interrenal is a 

 thin, elongated, yellow body with one or two anterior lobes. It 

 extends in the median line between the two kidneys or occupies the 

 internal portion of the ventral surface of each kidney. In Teleosts 

 the suprarenals, varying from two to five in number, are frequently 

 found embedded in the kidneys. 



Lymphoid Tissue. — Ordinary lymphatic glands are absent in 

 fishes and it is probable that functional lymphoid tissue abounds 

 to compensate. The large anterior portion of the mesonephros in 

 the Teleostomi, called the head kidney, is almost entirely replaced by 

 lyjnphoid tissue, practically no renal structure remaining. It is 

 concluded, from the presence of free red corpuscles and oxyhemo- 

 globin, that the head kidney performs a blood destroying function. 

 The spleen, of course, forms leucocytes and also destroys blood 

 corpuscles, devouring worn out reds. Possibly the head kidney 

 performs a similar double service. 



Fig. 141. Four-eyed fish, Anableps dovii. (Courtesy of Amer. Mus. of Nat. Hist.) 



Senses. — Fishes vary greatly in their use of the senses. Some 

 are predominantly olfactory or gustatory while others depend 

 largely upon vision to direct their movements. In all, the tempera- 

 ture sense ^ and lateral line sense are of paramount importance. 



Olfaction, Gustation. ( Chemical Sense.) ^ — Fish have a chemical 

 sense dependent on free nerve endings; smell, dependent on a highly 

 developed distance receptor, the olfactory nerve; and taste, which 



^ Chidester, F. E. 1924. A critical examination of the evidence for physical 

 and chemical influences on fish migration. Brit. Jour. Exp. Biol., vol. 1, pp. 79-118. 



* Parker, G.H. 1922, Smell, Taste and Allied Senses in the Vertebrates. J. B. 

 Lippincott, Philadelphia, Pa. 



