THE DEVELOPMENT OF ECHINOCAEDIUM COKDATUM. 477 
rods on each side and the lateral branches of the aboral rod 
(fig’- but the lower extremities of the body rods rest 
against the sides of the aboral rod without fusing with it 
(figs. 8, 9). 
'Idle little four-armed larva can now feed, and if placed 
under suitable conditions rapidly grows in size. Already by 
the end of the fourth day two additional calcareous centres 
appear, situated one on each side of the larva in the concavity 
between oral lobe and post-oral arms. Each of these centres 
has three rays — one directed outwards, one upwards and one 
downwards. From the outwardly directed ray there is de- 
veloped a latticed rod consisting of two ])arallel bars connected 
by cross-bars. This rod is the postero-dorsal rod; and as 
it grows it pushes out a corresponding protrusion of the 
ciliated band which is the postero-dorsal arm (p. cl. a., 
fig. 6). By the eighth or ninth day this arm has grown to 
two thirds the length of the corresponding post-oral arm and 
another calcareous centre has appeared. This latest centre 
is a median unpaired crescent situated on the dorsal side of 
the oesophagus at the base of the oral lobe. It is termed the 
dorsal arch [d. a., fig. 6) and in succeeding days it furnished 
by the growth of its two ends, two rods termed the prae-oral 
rods which form the skeleton of the two pr^e-oral arms. 
These last-named arms are situated in front of the mouth just 
beneath the antero-lateral arms and slightly nearer the 
middle line (pr. o. a., fig. 6), and it is a noteworthy circum- 
stance that whereas the other arms only appear when the 
corresponding rods of the larval skeleton impinge on the 
ectoderm, the rudiments of the pr^-oral arms appear before 
the branches of the dorsal arch reach them. The formation 
of the arm is certainly not due to a mechanical stretching of 
the ectoderm by the growing rod beneath it, for in unhealthy 
larvae the ectoderm can become contracted and the rod pro- 
jects from the apex of the arm as a bare spine ; we must 
rather picture the growth of the ectoderm which forms the 
arm as the response to a chemical stimulus emitted by the 
calcareous rod, and in the case of the prse-oral arms we must 
