No. 2.] DEVELOPMENT OF MANICINA AREOLATA. 213 
39, become the directive pairs, and on each side of the long 
axis of the cesophagus, 5 and 1 unite to form one pair, and 6 
and 2 another pair. 
In this connection it may be observed that after the larve 
became attached, the lumen of the cesophagus was decidedly 
bilateral in the living animal, and bore the relations to the 
mesenteries shown in Fig. 39. The shape of the lumen is, 
however, susceptible of great changes, and in many of the 
sections through attached larve the typical shape is not re- 
tained (Fig. 36), or the cesophagus is compressed in the (nor- 
mally) long axis, and drawn out in the short (Fig. 45). In the 
swimming larva the small size of the mouth will not permit one 
to decide as to the shape of the cesophagus in the living animal. 
After examining a large number of sections I conclude that in 
stages as old as Fig. 27, the arrangements are as in the attached 
larva: the lumen is bilateral and has one of the first pair of 
mesenteries on each side of its long axis. In earlier stages, 
such as Figs. 7, 14, 18, I could not decide whether the cesopha- 
gus had acquired its ultimate shape. 
We now come to the formation of filaments for the second 
and subsequent pairs of mesenteries. In Fig. 18 the reflected 
ectoderm which forms the cesophageal wall of the larger cham- 
ber extends horizontally from 1 to 2. The mesenteries of the 
second pair, 3 and 4, are extremely incomplete. In Fig. 27 these 
mesenteries are complete, and now the reflected ectoderm does 
not extend from I to 6, but is represented by the patch &.£. 
between 3 and 4. In sections of the same larva (Figs. 28 and 
29, below the level of Fig. 27) a certain irregularity makes its 
appearance, which is connected with the formation of the third 
mesentery, and of which I shall speak later. In the typical 
larva of this stage the sections below Fig. 27, between it and 
the lip of the cesophagus, are precisely like Fig. 27. This 
means that the mesenteries 3 and 4 are both complete through- 
out the length of the cesophagus, and that the cesophageal wall 
of the chambers inclosed between 2 and 4, and 3 and I respec- 
tively, is endodermal. At the very lip of the cesophagus in 
some larve the cesophageal walls of these chambers are ecto- 
dermic. ’ In a section just below the cesophagus it is found 
that the mesenteries 3 and 4 have short filaments. Further, 
the tract of ectoderm &.£. only extends upwards for about 
