No. 2.] DEVELOPMENT OF MANICINA AREOLATA. 218 
intermesenteric chamber. Referring the section to Fig. 27, 
the right half is through mesentery 4, and the left through the 
chamber opposite at about the point x. 
Having now described the ordinary way in which the first 
and second pairs of mesenteries and filaments are formed, I 
will take up an exception, which has more bearing on the rela- 
tionship between the Anthozoa and Scyphomeduse. Figs. 
30-33 are transverse sections, numbered from above down, of 
a larva in which the first pair of filaments extended about half 
the length of the body. The cesophagus throughout its verti- 
cal extent is apposed to the surface ectoderm over a wide tract, 
ato 6in Fig. 30. On running through the series of sections it 
becomes evident that the tract a—d is not the meridian of a sin- 
gle mesentery, as it would be in a normal larva like Fig, 14, 
but is the space between two mesenteries. The first section 
below the cesophagus, Fig. 31, shows that a rather wide lobe of 
ectoderm is growing down, and also that between a and @ this 
lobe has been forced apart from the body ectoderm. In Fig. 32 
(two sections omitted between 31 and 32) a lobe of endoderm 
‘has grown in between a and 4, and has thus given rise to two 
mesenteries, which are provided with a common filament. In 
the section below, Fig. 33, the lobe of endoderm is hollowed 
out, and the two mesenteries definitely established. In a sec- 
tion (not figured) below Fig. 33, the mesenteries @ and 0 exist as 
separate ridges, and the common filament has split into its con- 
stituent parts, 1 and 3. The filament 3 extends a very short 
distance down, and the mesentery 6 only reaches the equator of 
the larva. The mesentery a, with its filament 1, is the fellow of 
2, on the opposite side of Fig. 32. These two mesenteries 
belong to the first pair. I have assumed that a is the primary 
mesentery, since its filament is larger than that of 2. 
Bearing in mind the normal development as illustrated in 
Figs. 12 and 14, the exceptional character of this larva is due 
to the fact that one of the second pair of mesenteries is formed 
at the same time and in the same manner as the mesenteries of 
the first pair. This aberrant member of the second pair, 4, Fig. 
30, etc., may be called the third mesentery. We may suppose 
that the cesophagus, applying itself to the body ectoderm along 
the line not only of the first a, but also of the third mesentery 4, 
was unable to force down the endoderm in these lines without 
