270 FISHES CHAP. 



difference in physiological value between the two types must be 

 far more considerable than is indicated by a comparison of their 

 relative superficial areas alone. 



The evolution of the spiral valve was probably due to the 

 necessity of increasing the absorptive area of an almost straight 

 unconvoluted intestine, a result which in other animals is often 

 obtained by an increase in the length and concurrent convolution 

 of the intestine itself. Any attempt to correlate the variations in 

 the degree of perfection or imperfection of the valve considered as 

 an absorptive mechanism with any special variations in the nature 

 or quality of the food is, however, a very difficult problem, and a 

 satisfactory explanation has yet to be found. The difficulty, 

 moreover, is increased by the fact that the majority of Fishes 

 with a spiral valve are mainly carnivorous ; the Elasmobranchs, 

 in which this structure is at the same time most highly developed 

 and most variable, exclusively so. On the other hand, the term 

 " carnivorous " covers a multiplicity of minor differences in the 

 nature and relative digestibility of different forms of animal food, 

 and it is quite possible that it is with differences of this kind 

 that the specific or individual variations in the development of 

 the spiral valve are associated. The absence of the valve in the 

 variously nourished Teleosts, save perhaps as a vestige in one or 

 two, is also difficult to account for, although it is not improbable 

 that compensating structural modifications exist in this group. 

 As a rule, the intestine is much more convoluted in these Pishes, 

 but to an extent which varies greatly in different species, while 

 the characteristic pyloric caeca and the spiral valve appear to a 

 certain extent to be developed in inverse proportion to one 

 another. 



The Glands. 



The glands associated with the alimentary canal in different 

 Fishes are (1) the gastric glands, (2) the liver, (3) the pancreas, 

 (4) the pyloric appendages, and (5) the "rectal" gland. 



Oral salivary glands are wanting in all Fishes, the only 

 secretory structures in the mouth being numerous mucus-secreting 

 goblet cells, which here, as elsewhere throughout the alimentary 

 canal, are intermixed with the ordinary epithelial cells. 



The Gastric Glands. — The Gyclostomata and Dipnoi do 

 not possess any specially differentiated gastric glands, and it is 



