3284 



ECHINODERMS 



fly springs, so the sea urchin or the starfish egg gives rise to a larval form, in whose 

 body, as it were, the mature form is developed. The particular shape of the larva 

 varies in the different classes of echiuoderms; but the differences are not essential, 

 and it is clear that all the larval forms are modifications from one primitive type. 

 The changes passed through in the development of the common sea urchin {Strongy- 

 locentrotus dree bachien sis} are depicted in the illustration on p. 3285, in which the 

 drawings are very greatly magnified. 



DEVELOPMENT OF A SEA URCHIN. 



(Stage 10.) 



The fertilized egg divides and subdivides until a round ball of cells is formed. 

 This is then pushed in at one end, as one might push in a soft India rubber ball, 

 so that there is formed a little sac with a double wall to it (stages i, 2). Stage 3 

 shows this in outline as though transparent; and one sees the opening a turned 

 downward, and the canal d, which foreshadows the intestine. At the upper pole 



