Acanthurus triostegus sandvicensis — Randall 
227 
aspect of the digestion of these animals. Yonge 
( 1931 ) pointed out that information as to what 
an animal may collect and pass into its alimen- 
tary system may or may not indicate the true 
food of the animal. Thus, a study of digestion 
should logically accompany an investigation of 
food habits. The author is especially indebted 
to P. B. van Weel of the University of Hawaii 
for his counsel in the following research on di- 
gestion. 
Morphology of the Digestive Tract 
The similarity of the stomach of the manini 
to that of Acanthurus coeruleus as described by 
Breder and Clark (1947) has been mentioned. 
The manini stomach is elongate, and divisible 
into cardiac and pyloric portions. The cardiac 
part has prominent, longitudinal, irregularly 
scalloped folds on the inner surface; the pyloric 
part is smooth. A cross-section of the pyloric 
part showed the muscle tunic to be about the 
same width as the mucosa. There are few mul- 
ticellular glands. The stomach wall is less than 
0.1 mm. thick except posteriorly near the pylorus 
where it is slightly thicker than 1 mm. 
Just posterior to the pylorus are five pyloric 
caeca. Several authors (including Yonge, 1931) 
have claimed that the pyloric caeca has taken 
over the role of the pancreas in certain teleosts, 
the latter organ supposedly being absent in these 
fishes. Dawes (1929) found that the pyloric 
caeca of the plaice ( Pleuronectes plates sa) had 
the same structure as the intestine with which 
it was in free communication, and Rahimullah 
( 1945 ) came to the same conclusion after exam- 
ination of 119 species in 50 different families 
of fishes. The structure of the pyloric caeca of 
the manini is consistent with the findings of 
Dawes and Rahimullah. The bile duct opens 
into the base of one of the pyloric caeca. 
The pancreas of the manini was difficult to 
find. It was finally located in the form of two to 
four ( usually three ) small, round glands buff in 
color lying in the mesentery alongside the bile 
duct near its junction with the liver. 
The intestine of the manini is long, in keep- 
ing with the well-founded biological principle 
that herbivorous animals have lengthy intestines. 
The length of the alimentary tract of large adult 
manini is nearly six times the standard length 
of the fish. Most of this length is attributable 
to the intestine which is complexly folded within 
the body cavity. The length of the alimentary 
tract of small juvenile manini is only slightly 
greater than three times the standard length of 
the fish. The increase of the alimentary tract 
relative to standard length is shown in the graph 
of Figure 3. With increasing size the volume 
of a body requiring nutriment increases faster 
than the intestinal surface if the rate of growth 
of the two is equal. A disproportionate elonga- 
tion of the intestine is necessary to keep the 
area of absorptive surface adequate to the needs 
of the body. Hiatt (1947: 254, 257) noted a 
striking increase in the relative growth of the 
intestine of the herbivorous milkfish ( Chanos 
chanos) between 90 and 115 mm. standard 
length. The ratio of intestinal length to standard 
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Standard length fnm) 
FIG. 3. Change in length of the alimentary tract of 
Acanthurus triostegus sandvicensis with increase in 
standard length. The group of points at A represents 
the length of the tract of 10 specimens (24-27.5 mm.) 
in the acronurus stage which were taken at a night 
light offshore. The points at B represent 10 tide-pool 
specimens (24—28 mm.) which have just completed 
transformation from the acronurus to the juvenile 
stage. 
