312 
I’ROF. E. W. MACI5KIDE. 
vioiisly. 'J'liis extra rod is the “ antero-lateral rod” of 
the larval skeleton and the protrusion is the rudiment of the 
antero-lateral arm. Meanwhile the post-oral rod has 
pushed out a posterior protrusion of the ectoderm on each 
side just above the blastopore, now become the anus. This 
protrusion is the rudiment of the post-oral arm (called 
‘^anal arm” by Driesch and Herbst). Up till now the larva 
has been uniformly ciliated all over, with a tufc of specially 
long cilia at the anterior pole, but now the cilia become re- 
sti'icted to a ridge forming the edge of the concave ventral 
surface. This is the longitudinal ciliated band of the 
larva, and it runs along the edges of the protrusions which are 
the rudiments of the larval arms. 
The tuft of specially long cilia becomes incorporated in the 
anterior border of this band. 
A dorsal view of a larva in this stage is given in PI. 14, 
fig. 4. As soon as the alimentai’y canal is complete spots of 
orange- red pigment appear just under the ectoderm all 
over the larva. The pigment is carried by wandering 
mesenchyme cells, which can be seen to migrate into the 
ectoderm and to discharge their pigment (which is probably 
of an excretory nature) to the exterior in the form of granules. 
During the next day the antero-lateral and post-oral arms 
grow longer and an adoral band of cilia (ad., fig. 5) becomes 
well defined. This consists of two ridges of thickened epithe- 
lium lying in the ventro-latei*al walls of the larval oesophagus 
belonging partly to the ectodermal and partly to the endodermal 
region of this, which carry long cilia. There is reason 
to believe that these cilia produce an outwardly 
directed current and that their function is to re- 
move excess of food matter from the i-egion of the 
mouth. At least if a living larva be watched, particles sus- 
pended in the water shoot out violently from the ventral side of 
the mouth. On the left side the madreporic pore-canal and pore 
are formed by the union of a dorsally directed up-growth from 
the left coelomic sac and a slight in-pitting of the ectoderm. 
Constrictor m use 1 es, which cause the oesophagus to exe- 
