238 
E. W. MACBRIDE. 
on eacli side of the larva are the two ciliated epaulettes, 
which at one time formed part of the band, but which have 
become completely separated from it, and serve as main loco- 
motor organs for the larva in this stage of its development. 
One on each side, the dorsal, is situated in front of the postero- 
dorsal arm, and one, the ventral, is behind the post-oral arm. 
Between them the ciliated band dips down in a deep loop. 
Within the curve of this loop is the “ echinus-rudiment” on 
the left side, consisting of the hydroccele, which appears as 
a flattened, thick-walled vesicle, with an almost virtual lumen 
and an ectodermic invagination — the amniotic cavity, the 
inner end of which is pressed against the hydroccele. The ab- 
normality of the larva consists in the fact that a 
precisely similar “echinus-rudiment” is situated 
on the right side, where in the normal larva at this stage 
one would expect to find nothing of the sort. Through the 
transparent dorsal integument the larval oesophagus and 
stomach can be seen. Both are perfectly normal. At the 
sides of the oesophagus can be seen the thickenings which 
carry the two halves of the adoral band of cilia (ad., i). 
External to this thickened band on each side can be seen 
the outline of the anterior coelom ( l.a.c ., r.a.c.). Near the 
middle line above the stomach can be seen a small circular 
pore ( m.jp .). This is the primary madreporic pore. 
Now when treating of the normal development of Echinus 
I have shown (5) that after the coelom has been separated 
from the archenteron it divides into right and left sacs, and 
that the left sac acquires a communication with the exterior 
by the formation of a vertical canal, the pore-canal, which 
fuses with an ectodermic invagination. Subsequently each 
sac divides into anterior and posterior portions and the pore- 
canal communicates with the hinder part of the anterior 
portion of the coelom on the leftside. The anterior coelomic 
sacs apply themselves to the larval oesophagus, to which they 
supply muscles, whilst the posterior sacs come to lie at the 
sides of the stomach. From the liindermost portion of the 
left anterior sac a vesicle is partially constricted off; this is 
