No. 2.]| DEVELOPMENT OF MANICINA AREOLATA. 223 
with Fig. 26 (magnified to the same degree) it is seen that the 
mesentery has become more elevated, and at the same time 
thinner ; also, that where the endoderm cells end and the filament 
cells begin there has been a pinching in, which, added to the 
actual bulging out of the filament, has very distinctly worked 
off the latter from the mesentery. The bulging out of the fila- 
ment is due to the lateral expansion of the axial jelly shown in 
Fig. 26. The expansion of the jelly has so taken place that the 
filament, Fig. 35, is divided into three portions: the main body 
to which are confined the nettle and gland cells ; and two tracts, 
v./, which may be called the ventro-lateral tracts. The latter 
are composed exclusively of slender supporting cells. As in 
the swimming larva, the filament cells are sharply marked off 
from the cells of the mesentery. In spite of the twisted con- 
dition of the lower part of the filament, sections show that the 
structure is the same as in the upper part. 
In the adult the upper portion of the filament on a complete 
mesentery is comparatively straight, but the main portion is 
twisted, Fig. 50. The filaments of the incomplete, are less 
twisted than those of the complete mesenteries. The filaments 
are attached their whole length to the mesenteries, there being 
no free acontium ; they are, however, capable of extrusion both 
through the mouth and (though I could not find the apertures) 
through pores (cynclides) in the body wall. 
Unlike the larval filament, that of the adult has a different 
structure at different levels. Figs. 52, 53, and 54-are sections 
through the different parts of a filament on a complete mesen- 
tery. Fig. 52 is through the upper third. The filament itself 
has almost exactly the shape shown in Fig. 35, but the mesen- 
tery is swollen out and forms two lateral lobes, ./., between 
which the filament rests. The ventro-lateral tracts are much 
better marked in the adult than in the larval filament. This is 
due to the continuation of the pinching in process which had 
already gone some distance in Fig. 35, and to the outgrowth 
of the mesenteric lobes, m./. By these means the slender 
“waist” is produced, which indicates the separation of filament 
and mesentery. 
The ventro-lateral tracts of the filament, both in the upper 
portion of its course, Fig. 52, and lower down, Figs. 53 and 54, 
are made up exclusively of supporting cells. The main body of 
