PYRG) 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 Fishes, 
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 
oe are intermixed with the ordinary epithelial cells. 
The Gastric Glands.—The Cyclostomata and Dipnoi do 
not possess any specially differentiated gastric glands, and it is 
