482 
E. W. MACBRIDE. 
neural veil whicli is only perforated some days later. Tlie 
outline of tlie urchin is ellipsoidal, not circular as in the imago 
of Echinus, and whereas in Echinus the oral surface is flat 
and the five primary tube-feet are the principal organs of 
locomotion, in Echinocardium on the contrary the young 
urchin moves about by the aid of its spines and the oral 
surface is rounded. A curious consequence of this difference 
in mode of locomotion is seen in the condition of the water- 
vascular system in the late larvm and young imagines of the 
two genera. In Echinus this system is gorged with fluid 
for the tentacles are very extensible. In Echinocardium the 
lumina of the water-vascular ring and tentacles are very 
narrow since the tentacles are only extensible to a slight 
extent. 
There is another great difference between the young 
imagines in the case of the two genera. In Echinocardium 
the five primitive tube-feet, which, it must be remembered, 
represent the apical terminations of the radial canals, arise 
from near the mouth, and all the spines are situated further 
from the mouth, and apparently must belong to the apical 
region of the fully developed adult. In Echinus, on the 
conti-ary, the tube-feet arise from the edge of the oral surface 
of the young imago, and the pointed spines are situated as 
near or nearer to the mouth, and hence belong to the region 
of the corona in the fidly developed adult. Only the square- 
topped spines which arise outside the amniotic area belong to 
the apical region of the fully developed adult. Hence the 
conclusion inevitably follows that the floor of the amniotic 
cavity does not represent the same region in the two genera, 
and the speculation is awakened that perhaps in the far-off 
common ancestor of Echinus and Echinocardium no 
amnion was developed at all, but the spines and tube-feet of 
the adult were developed in an exposed condition on the side 
of the larva, as is the case in Asteroidea. 
Most of the spines of the imago of Echinocardium are 
conical, but there occur at one end of the ellipsoidal imago a 
short crescent of shorter flattened club-sbape spines {cl. sp., 
