22 CHILDHOOD OF ANIMALS 



change to the adult condition is a condensed and rapid repetition 

 of the slow ancestral history. 



The forms of larvae and the kinds of metamorphoses which occur 

 in marine invertebrates are many and varied, and the few examples 

 I shall choose will serve, I hope, rather to show the interest and 

 difficulty of the subject than to beguile readers into thinking they 

 or I understand it. Echinoderms, of which we all know starfish 

 and brittle-stars, sea-urchins and sea-cucumbers, crawl at the 

 bottom of the sea and show a radiate, generally a five-rayed, sym- 

 metry. That is to say, the organs of the body are arranged round 



FIG. 10. Three stages in the metamorphosis of the Sole. (After FABRE- 

 DOMERGUE and BIETRIX ; slightly enlarged.) 



a central axis, which is short in the flat echinoderms; such as the 

 starfish and brittle-stars, or long in the globular and oblong ones, 

 such as the sea-urchins and sea -cucumbers, like the spokes of a 

 wheel or the petals of a five -rayed flower. The eggs of most of 

 these echinoderms are very small, and soon after they are shed into 

 the water grow into little floating larvae. The larvae quickly assume 

 the shape of a thick -walled cup, the outside of which is covered with 

 small, waving threads of living matter, called cilia, and the hollow 

 of which forms the primitive digestive cavity. The cup grows larger 

 and longer, and its aperture narrows to a small pore. A new aperture 

 breaks through into the digestive cavity and becomes the mouth ; 

 the original aperture sometimes closes up, sometimes remains to 

 form the posterior aperture of the digestive canal. The larva 

 changes its shape, becoming flat, or even concave, on the side where 

 the mouth and anus lie, and remaining dome -shaped on the other. 



