DEVELOPMENT OF ECHINODERMS 



3285 



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 

 downward 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 processes (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 carbonate of lime, which by and by grow 

 into the skeleton of the larva, in shape something like an inverted easel. The two 

 lower ciliated bands now grow toward one another, so that the vent comes to lie be- 

 neath them (stages 7 and 8). They also join themselves to the two upper bands, so 

 that there is formed a single zone of cilia, which persists to the end of the larva's 

 life. Already can be distinguished the beginnings of the apex and of the pro- 

 cesses (tf), which finally lengthen into 

 the arms that give such a strange 

 appearance to the larva, in the sea 

 urchins and also in the starfish and 

 brittle stars. In stage 8 can be seen, 

 at b, the pore that admits water to the 

 water-vascular system ; and at this 

 point will lie the madreporite of the 

 future sea urchins. The next illus- 

 tration shows all these parts in a 

 rather more advanced stage: a is the 

 vent; c, the hinder intestine; d, the 

 stomach, around which a deposit of 

 spicules indicates the first beginnings 

 of the body of the sea urchin; o, the 

 gullet; m, the mouth; <?, the arms of 

 the larva; r, the calcareous rods that 

 support them; v, two more strongly- 

 developed and slightly-projecting por- 

 tions of the ciliated band; and w, the 

 water vessels. The larva in its full 

 development is shown in the illustra- 

 tion on p. 3281, in which the letters 

 have the same meaning. This larval 

 form is called a Pluteus, on account of 

 its frequent changes of shape, as it 

 swims about with its arms constantly 

 moving. It will be noticed that 

 through the whole of its development 

 it retains a two-sided symmetry, such ^^ ^ URCHIN (StrongyloceHtrotia) . a . Frombelow; 



that if CUt down the middle it WOUld *. From above (enlarged 20 times). 



