212 WILSON. [Vot. II. 
VI. MESENTERIES AND FILAMENTS FROM THE SECOND TO THE 
SrxtTH PArr. 
Before proceeding to the detailed description of the reflection 
of ectoderm, which, as I conclude, leads to the formation of all 
the filaments except the first pair, it will be convenient to give 
an account of the order in which the mesenteries appear. 
The first appearance of the second pair of mesenteries has 
already been described, Fig. 18. In a later stage, Fig. 27 (trans- 
verse section) this pair is complete at the level of the figure. 
At a lower level, just above the lip, Fig. 28, one of the pair, 3, 
is still complete, while the other, 4, is incomplete. The dif- 
ference in growth between the two mesenteries of the second 
pair is an exception; they usually develop at the same rate. 
Below the cesophagus, Fig. 29, both mesenteries exist as slight 
ridges, which extend the length of the larva. 
The third pair of mesenteries have appeared in Fig. 27. They 
remain insignificant during the swimming life of the larva. 
After the larva has become attached, the third pair is promi- 
nent ; and the fourth pair is also found, in the position shown in 
Fig. 34 (a transverse section taken below the cesophagus — the 
primary mesenteries are supplied with large and coiled filaments). 
According to Lacaze Duthiers, the fourth pair appears between 
the first and second pairs. The Hertwigs suggested (?) on gen- 
eral grounds of symmetry that the order of appearance was as I 
have figured. As regards the fifth and sixth pairs, however, the 
old account of Lacaze Duthiers holds for JJanicina as against 
the figures given by the Hertwigs for Adamsia (Taf. 1, f. 3). 
This is seen on referring to Fig. 39, a transverse section through 
the cesophagus of a stage with twelve mesenteries. The fifth 
and sixth pairs develop simultaneously on opposite sides of the 
primary mesenteries. In Adamsza, according to the Hertwigs, 
the two pairs of mesenteries appear on opposite sides of the 
long axis of the cesophagus, in the chambers between the first 
and second pairs. 
Fig. 39 represents my oldest larval stage, and I was conse- 
quently not able to trace the development of the muscle plates 
and the rearrangement of the twelve primary mesenteries. 
According to the accepted account, the pairs 3 and 4, in Fig. 
