3 l6 
ECHINODERMS. 
shows this in outline as though transparent; and one sees the opening a turned 
downwards, and the canal d, which foreshadows the intestine. At the upper pole 
DEVELOPMENT OF A SEA-URCHIN ,Stages 1-8). 
of the embryo, near the number 3, is a small 
tuft of cilia, by the motion of which the embryo 
swims about. In stage 4 this ciliated area is 
seen to have extended downwards to the letter v. 
The intestine now develops in such a way that 
the original opening (a) remains as the vent, the 
middle part ( d ) widens into a stomach, and a 
fresh mouth-opening (m) is pierced through at 
its upper end. This is seen from the side in 
stage 4, and from the front in sta^e 5. But 
before the mouth is formed, two ear-like pro¬ 
cesses ( w ) show themselves, which are important 
as being the beginnings of the ambulacral and 
water-vascular systems. There also appear a 
few delicate, symmetrically laced rods of car¬ 
bonate of lime, which by and by grow into the 
skeleton of the larva, in shape something like 
development of a sea-urchin (stage 9). an inverted easel. The two lower ciliated 
