No. 2.] DEVELOPMENT OF MANICINA AREOLATA. 217 
along its free edge as a tiny filament (Fig. 38, left half; the 
right half is through chamber c¢, Fig. 39). The lobe for the 
third pair of filaments is almost always present in larvze with 
eight mesenteries: in Fig. 36, a section just above the lip, it is 
marked ¢. In this larva the mesenteries of the second pair are 
backward in development ; they are not yet perfectly complete, 
though on one of them a very small filament is seen. 
As the fourth pair of mesenteries continue to increase in 
size, the tract of ectoderm, which belongs to the chamber «¢, in 
Fig. 39, extends farther upwards. In Fig. 39 it reaches about 
the same level as in Fig. 27 (R.£.). In a slightly more ad- 
vanced stage, Fig. 38 (right half), it extends nearly the whole 
length of the cesophagus ; and in the series of transverse sec- 
tions, Figs. 43-45, it likewise extends to nearly the uppermost. 
limit of the chamber. The latter series is a very good one, as 
the polyp was killed in its natural shape. In life, when the 
young coral is expanded, there is a very distinct oral cone, in- 
dicated by the line m-m in Fig. 38. In dying, the oral surface 
is almost always retracted, and the median longitudinal section 
is then as in Figs. 37 and 38. The transverse section, Fig. 43, 
is taken through the line z-y, in Fig. 38. Figs. 44 and 45 lie 
above it. In the larva from which the series was taken, both 
the third and fourth pairs of mesenteries were very well devel- 
oped below the level of z#-y, but above this level they were not 
perceptible. Judging from all my other sections I should say 
that this rapid development of the mesenteries along the side- 
wall of the polyp, as contrasted with their backwardness in 
the upper cesophageal region, was exceptional. In the sections 
figured the first and second pairs of mesenteries are complete. 
The chambers a and 8, together with c, represent the larger 
of the two primary chambers. The walls of @ and @ are endo- 
dermal except for a short distance above the cesophageal lip, but 
in ¢ and in d (the smaller of the two primary chambers) the re- 
flected ectoderm extends very far up. As the third pair of 
mesenteries lie in chamber ad, the ectoderm of this chamber, as 
might be expected, extends farther up than in ¢, in which lie 
the fourth pair. In the ectoderm of d there are a few large 
nettle cells, a rare occurrence. At the upper limit of each tract 
of ectoderm, the overlapping of the layers, previously referred 
to, is shown. This overlapping of the layers, while uncommon, 
