222 WILSON. [Vot. II. 
VIII. HisToLoGicAL STRUCTURE OF THE FILAMENTS. 
The very young mesenteric filament is shown in cross-section 
in Fig. 17. In this larva, the first pair extend about half the 
length of the body. In Fig. 29, though the mesentery is ele- 
vated above the general endoderm, the filament retains its simple 
character. It is roughly hemispherical in section, and is sepa- 
rated from the mesentery by a thin sheet of supporting mem- 
brane. Besides supporting cells, there are present granular 
gland cells. In most larve in which the first pair of filaments 
reach the aboral end of the body, the filament is no longer 
separated from the mesentery by supporting lamella. Fig. 19 
gives a surface view of such a larva, and Fig. 26, a cross-section 
of one of the long filaments. In the latter figure the sheet of 
supporting lamella, on which the filament formerly rested, has 
given place to an accumulation of jelly at the apex of the mes- 
entery. Though the cells of the filament are practically con- 
tinuous round this gelatinous axis with the cells of the mesen- 
tery, the line of demarcation is very evident on each side owing 
to the different histological characters of the endoderm and 
ectoderm cells. In the filament there are now numbers of large 
nettle cells, and the gland cells are far more numerous than in 
earlier stages. Nervous elements are very probably present, as 
there is a granular stratum in the deepest part of the filament. 
As is shown in Fig. 19, the filament increases in size towards 
its lower end. Near the close of the swimming life the first 
pair of filaments begin to get slightly curved and twisted in the 
lower part of their course. The young filament before its cells 
become continuous with those of the mesentery, is very loosely 
attached to the latter; in a number of cases I observed that the 
mesentery and filament had entirely separated from each other 
(Fig. 29, Mes. 3), owing, no doubt, to the contraction caused by 
the killing fluid. 
Fig. 35 is through one of the first pair of filaments, and its 
mesentery, of an attached larva with eight mesenteries. In the 
attached larva these filaments pursue a straight course for a 
short distance below the cesophagus. In the lower part of their 
extent, they are curved and twisted as in the adult, and in this 
region sections like Fig. 34 are obtained. Fig. 35 is through the 
straight portion of the filament. On comparing this figure 
