476 
E. W. MACBEIDE. 
whilst the rest of the archenteron which forms the gut, 
becomes mai-kecl oat by coastrictions into oesophagus, stomach 
and intestine. Between the rudiments of the post-oral arms 
and the part of the body into which the nascent antero-lateral 
rods project there appears a concavity. This concavity 
causes the part of the body supported by the antero-lateral 
rods to stand out prominently and we shall call it the oral 
lobe (o. figs. 3, 4, and 5) . The cilia which clothed the whole 
surface of the gastrula become now confined to a thickened 
band of epithelium which runs along* the edge of the oral 
lobe and over the tips of the post-oral arms. This is the 
longitudinal ciliated ban d, the order of locomotion in 
the larva (/. ciL, fig. 3). 
On the inner aspect of the oral lobe the stomodseum [stom.f 
fig. 4) appears as a shallow depression; this soon becomes 
connected with the endodermal oesophagus and the alimentary 
canal is thus completed ; the blastopore of course persists as 
the anus. All the changes are completed by the end of the 
second day. 
Along the sides of the compound oesophagus a band of 
thickened epithelium makes its appearance. This is the 
rudiment of the adoral ciliated band [ad. ciL, fig. 6). 
During the third day the new centre of calcification mentioned 
above develops rapidly. From one of its arms there is deve- 
loped a latticed rod consisting of three parallel rods bound 
together by cross bars, which projects backwards and pushes 
out a corresponding club-shaped protrusion of the body-wall 
which is the well-known aboral spike of the spatangoid 
larva. The latticed rod is known as the aboral rod [ab. r., 
fig. 5). The antero-lateral rods also develop rapidly and push 
out a pair of protrusions of the ciliated band which are the 
rudiments of the antero-lateral arms (a. la., fig. 5). From 
each antero-lateral rod there is given ofl: a poscerior branch 
which runs back towards the lower pole of the larva parallel 
to the body rod. This branch is termed the recurrent rod 
(>•. r., figs. 4 and 5). In order to support the heavy aboral 
rod connections are formed between the recurrent and body 
