246 
E. W. MACBR1DE. 
deepens and becomes completely closed off from the exterior; 
the hydrocoele becomes five-lobed (only two of these lobes are 
represented as cut by the sections), the free ends of these lobes 
grow and protrude into the amniotic cavity, and in this way 
the unpaired tube-feet are formed. Between two adjacent 
tube-feet an ectodermic fold is developed from the floor of 
the amniotic cavity; this is the epineural fold. Its crest 
sends out two flaps which meet those of the other epineural 
folds, and thus the original floor. of the amniotic cavity is 
completely covered in by a new fold which forms a new floor 
at a higher level, and eventually forms the free surface of the 
ventral part of the young sea-urchin. 
The Avail of the left posterior coelom of the larva, Avhich 
impinges on the inner side of the hydrocoele, now begins 
to undergo changes. Opposite the base of each epineural 
fold, an evagination of the coelom is formed. This is the 
rudiment of one of the five tooth -pockets or dental sacs, 
which together form Aristotle’s lantern. The evagination 
is cut off from the rest of the coelom. Then its external 
Avail becomes thickened and projects into its cavity. This 
projection is the rudiment of the tooth. From the sides 
of the tooth-socket canals are given off which insinuate 
themselves between the lobe of the hydrocoele and the 
original floor of the amniotic cavity, now roofed over by the epi- 
neural false floor. These canals are the radial perihaemal 
canals, and the portion of the original floor of the amniotic 
cavity lying above them becomes conA r erted into the radial 
n er ve-cord of the adult. It will thus be seen that the groAvth 
of the “ Echinus-rudiment ” is an extremely complex process, 
involving not only the hydrocoele and the ectoderm above it, 
but also the external wall of the left posterior coelom, and 
finally, when the adult oesophagus is formed, the external 
wall of the larval stomach also. The calcigenous tissue Avliich 
fills the interior of the spines Avhich are placed in the intervals 
or inter-radii between the impaired tube-feet is mesenchyme 
budded from the Avail of the left posterior coelom, Avhilst the 
external covering of the spines is, of course, ectoderm from the 
