316 



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 (stage 9). 



DEVELOPMENT OF A SEA-URCHIN v 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 stage 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 

 an inverted easel. The two lower ciliated 



