16. 
ire 
18. 
25. 
27. 
28. 
MEM. M. C. Z., IX. No. 2.—ECHINODERMATA. 23 
Twenty-two hours after fecundation ; the embryo has become greatly lengthened, and is cylindrical. The cavity 
(d) has slightly expanded at the closed extremity, and the walls of the embryo are somewhat reduced in thick- 
ness except at the perforated region ; the body is somewhat translucent, slightly tinged with ochre-color. 
The opening () still serves as a mouth, although in more advanced stages a second opening is formed 
which becomes the true mouth of the embryo, at which time the present mouth then becomes the anus. 
Slightly older embryo than fig. 16, seen from the side ; the digestive cavity (d) is no longer in the axis, but 
is bent to one side (the lower side). 
Older embryo, seen in profile ; the pouch formed at the end of the closed extremity of the bent digestive cav- 
ity (7) is nearer the lower side than in fig. 17, and is moving towards the slight depression (m, the future 
mouth) placed in the middle of the larva. 
. A larva somewhat more advanced, seen in profile, in which the terminal pouch of the digestive cavity has 
actually come into contact with the wall of the lower side at m. The dorsal region of the perforated 
extremity projects slightly beyond the depression in which m is placed. The digestive cavity is not yet 
divided into distinct regions. 
. A larva somewhat more advanced (end of the second day), seen in profile. The digestive cavity is no longer a 
simple bent tube, as in fig. 18; it is strongly contracted near the extremities ; at the distal extremity two 
diverticula have formed, projecting upwards (w) ; a second opening (m) has been formed at the point of con- 
tact of the former closed extremity of the digestive cavity with the lower side; this connects the cesoph- 
agus, by a short tube, with the main pouch of the digestive cavity. This second formed opening (7) is the 
true larval mouth, while the first formed opening (z) now becomes the anus, after having, up to this stage, 
performed the functions of both mouth and anus. 
. Isolated digestive cavity seen from below, showing the position of the mouth and anus on the same side of the 
larva. The anal extremity of the larva bending over as in fig. 24 at about this stage, thus bringing the 
anal opening from the extremity of the larva to the lower side. The two diverticula (w w/) of the digestive 
cavity (the future water-tubes) are so far differentiated as to be quite distinct from the digestive cavity. 
The walls of these diverticula are excessively attenuated, and are scarcely connected with the digestive 
cavity. 
Larva somewhat older than stage of fig. 21, seen from above, in which the two small bodies, w, w!, the diverti- 
cula of younger stages formed from the pouch of the digestive cavity at its closed extremity (the problem- 
atic bodies of Miiller), have entirely separated from the digestive cavity from which they were formed ; the 
three divisions of the original cavity into intestine, stomach, and cesophagus are plainly marked out. 
. Older Jarva, scen from below at the end of the third day after fecundation, showing the triangular shape of 
the mouth (7m), the greater size of the problematic bodies w, w! (the water-tubes), which increase inde- 
pendently and at an unequal rate ; the tube w/ communicates with the madreporie opening (}) ; it also 
shows the position of the rudimentary oral and anal vibratile crescent cords. 
. The same as fig. 23, seen in protile, to show the position of the mouth in a strongly marked depression, the 
great increase in size of the oral part of the cesophagus, the swelling of the stomach, and the bending of 
the extremity of the intestine back and downward toward the mouth, so as to make a small angle with 
the trend of the stomach. 
Slightly older larva, seen from above. The principal difference between this stage and the preceding one con- 
sists in the greater increase in size of the vibratile crescents, which now form two small plastrons, and the 
greater size of the water-tubes. The intestine also bends so as to make, when seen in profile, almost a 
right angle with the stomach, which is pushed out farther toward the anal extremity. 
- More advanced larva, seen from the left profile, in which the oral pouch has assumed its characteristic pistol- 
shape. The stomach and intestine make a sharp angle with each other, the latter being much longer than 
the stomach proper. In its present aspect it closely resembles a retort, the stomach being the receiver, the 
intestine the tube. The anal and oral vibratile crescents are greatly extended towards the extremity of the 
body, the one on the oral, the other on the dorsal side. 
A larva six days after fecundation, seen from the right profile ; the water-tubes extend beyond the opening of 
the mouth, the tube leading from the dorsal water-pore (madreporic body) to the water-tube (w!) is quite 
distinctly seen. : 
The same larva as fig. 27, seen from below, showing the intestine thrown to one side of the axis of the larva, 
the water-tubes extending along the sides of the stomach toward the anal extremity. 
29-34. Asteracanthion pallidus. From Alexander Agassiz, Embryology of the Starfish, 1864 (Agass. Cont. Nat. 
29. 
30. 
Hist. U. S., V.), Pls. I1I., I1V., VII. Memoirs Mus. Comp. Zool., 1877, V., No. 1, North American Star- 
fishes, Pls. III., 1V., VII. 
Larva seen from the right profile, somewhat more advanced than any larva of 4. berylinus raised by artificial 
fecundation. 
The same larva seen from the oral side. The water-tubes have greatly increased in diameter ; they have 
united beyond the mouth, and also extend along the sides of the stomach so as to meet, but without unit- 
