TWO ABNORMAL PLUTEI OF ECHINUS. 
243 
the larva takes on a discoid form; this disc is the oral disc 
of the young Echinus formed out of the floor of the 
amniotic cavity. The ciliated band of the larva is seen to 
make a wide sweep round it, which is merely the enlarged 
re-entrant loop, which, in an undeveloped state, was noted in 
the larva of Echinus miliar is. On this loop can be seen 
the reduced stumps of the postero-dorsal post-oral arms. 
Outside it can be seen the dorsal ( d.ep .), and ventral ( v.ep .), 
and posterior ciliated epaulettes (p.ep.). The ventral ciliated 
epaulette is bent on itself in the shape of a U ; this is 
almost certainly a result of the contraction caused by the 
preservative (spirit) used. Turning our attention now to 
the disc of the “ Echinus-rudiment,” we notice the five 
primary unpaired tube-feet (pod.). Each is flanked by a pair 
of smaller tube-feet (pod. 1 ) ; these are the first paired tube-feet 
and eventually become the oral or gustatory tube-feet of the 
adult, whereas the primary tube-foot is destined to degene- 
rate into the insignificant knob in which the radial canal 
terminates. In the centre of the disc of each primary tube- 
foot is a pointed sense-organ, consisting of elongated 
ectoderm cells. In the intervals or inter-radii which 
intervene between adjacent primary tube-feet there are 
groups of spines — four in each inter-radius constructed on 
the same model as the spines of the adult; in fact they are 
the first adult spines. Each consists of a shaft supported 
by a fenestrated calcareous rod, attached to a boss by a 
thick collar consisting of radiating muscles. All the calcareous 
matter had been dissolved away when the larva reached me, 
but the calcigenous cells (calc.) can be seen through the 
transparent ectoderm. It will be noticed that the four spines 
are arranged in the form of a lozenge in each inter-radius. 
Withiu the circle formed by the tube-feet and the spines a 
circular band is to be seen; this is the n erve-ri n g of the adult 
shining through (nerv.). Within this again a pentagonal 
figure, subdivided into five secondary pentagons, is situated. 
These I interpret as the five dental pockets which together 
constitute Aristotle’s lantern, and within these the adult 
