414 WILSON. [ VOL. ET 
ture as the “ larval gland-cells ” of Clepszne (Whitman, No. 52, p. 157), though 
they are not pigmented and have a somewhat different position. 
The later differentiation of the pharynx is effected by an enormous thicken- 
ing of its mesoblastic investment on the dorsal side, the ventral mesoblast 
remaining extremely thin. 
3. Proctodeum.— The proctodzum is very long delayed in 
development, only making its appearance near the end of feetal 
life, when the body has an elongated form and concrescence of 
the germ-bands is nearly complete. It is formed as a hollow 
invagination of ectoblast which pushes its way between the 
hinder ends of the mesoblastic bands, and soon fuses with the 
archenteric wall. I have not been able certainly to determine 
the important question of its precise morphological relation to 
the ends of the germ-bands, owing to the fact that before its 
formation the primary mesoblasts break up into smaller cells 
and thus render the hinder limits of the bands somewhat diffi- 
cult to determine. In Polygordius, and many other annelids, 
the proctodzum is formed behind, or dorsal to the primary 
mesoblasts, z.e. outside the mosoblastic ring formed by the 
germ-bands; and this seems to be the typical position of the 
anus in annedids generally. In Lwmbricus towards the end of 
foetal life the primary mesoblasts break up intoa group of cells 
from which the mesoblastic bands (now entirely united along 
the ventral line) extend forward (Fig. 97). As the proctodzum 
grows inwards this mass of cells grows upwards at the sides of 
the proctodzeum in two horns, which finally unite in the dorsal 
line above it (cf Figs. 95-97). Thus the mesoblastic ring ulti- 
mately comes to surround the proctodzeum behind, as it does at 
a much earlier period the stomodzeum in front, the middle por- 
tion of the ring having meanwhile undergone concrescence 
throughout its whole extent. The stomodzeal invagination has 
the form of a longitudinal slit which at first faces upwards 
owing to the curvature of the body, but gradually is turned back- 
wards as the body straightens out. Its walls fuse completely 
with those of the archenteron and assume the same histological 
character, and every trace of the limit between them disappears. 
It is therefore impossible to determine how far forward the 
proctodzeum extends, but its extent is certainly very limited. 
