62 TROPICAL PACIFIC ECHINI. 
(Pl. 10, figs. 1, 2, 4, 5), at a sharp angle. In the young, the oesophagus nearly 
reaches the madreporic plate, with the axial organ on its right side (PI. 10, 
fig. 3, Pl. 11, figs. 8, 9). It is held in place with the flat almond-shaped actinal 
intestinal appendage by a mesentery which is fastened to the abactinal part of 
the left anterior interambulacrum, close to the odd anterior ambulacrum; this 
mesentery divides the upper anterior cavity of the test into two open chambers 
(Pl. 10, fig. 4). The free vertical edge of this mesentery is slightly folded near 
the genital ducts, broadening as it comes to the buccal membrane where it is 
apparently connected with the nerve-ring by what may be the lower part of the 
axial organ (PI. 10, fig. 5). A nerve ascends along the free edge of the mesentery 
and shortly before reaching the apex, connects with the axial organ. 
In young specimens, test without spines, 5 mm. long (PI. 9, figs. 4, 5, 6), the 
general shape of the alimentary canal is nearly like that of the adult. The actinal 
loop is broad where the oesophagus and actinal intestinal appendage are inserted 
(Pl. 8, figs. 3-5, Pl. 9, figs. 7, 2), and continues so until in its curve backward 
and to the right, it passes the right posterior ambulacrum. There it gradually 
narrows, until it reaches the anterior margin of the right anterior interambulacrum 
where it curves abruptly upward and becomes the abactinal loop, following the 
ambitus on the right side of the test. When it reaches the odd posterior inter- 
ambulacrum, it broadens and has a reéntering curve for passing the rectum 
(Pl. 8, figs. 3-5), in front of the median line; its outer edge being closer to the 
test, than that of the actinal loop. In the left anterior interambulacrum, the 
abactinal loop suddenly tapers (Pl. 8, figs. 4, 5, Pl. 9, fig. 2), passes upward 
close under the reproductive organs, and becomes tubular. It is held by mesen- 
teries to the abactinal median line of the odd posterior interambulacrum clear 
to its end at the anal plastron. Posterior to the oesophagus from the buccal 
membrane (PI. 10, fig. 5), there rises a narrow mesentery which soon turns a 
half spiral, with its broadest surface attached to the inner edge of the left half 
of the actinal loop, and to the actinal intestinal appendage; while the narrow 
part is fastened to the inner edge of the right half of the actinal loop (PI. 8, 
fig. 3, Pl. 9, fig. 1), as far as the odd anterior ambulacrum. Another mesentery 
is attached in a similar way to the abactinal loop of the alimentary canal (PI. 10, 
fig. 4), but is connected with some of the plates of the apical system. 
Close to the left anterior inner side of the actinal loop, on the mesentery 
is a brownish pigmented band (PI. 8, fig. 3; Pl. 9, fig. 7), leading from the inser- 
tion of the oesophagus, and extending posteriorly. This is probably the acces- 
sory intestine. 
