CHAPTER XX. 



ECHINODERMATA 1 . 



THE development of the Echinodermata naturally falls into 

 two sections: 



(i) The development of the germinal layers and of the 

 systems of organs; (2) the development of the larval appendages 

 and the metamorphosis. 



The Development of the Germinal Layers and of tJie Systems 



of Organs. 



The development of the systems of organs presents no very 

 important variations within the limits of the group. 



Holothuroidea. The Holothurians have been most fully 

 studied (Selenka, No. 563), and may be conveniently taken as 

 type. 



The segmentation is nearly regular, though towards its close, 

 and in some instances still earlier, a difference becomes apparent 

 between the upper and the lower poles. 



At the close of segmentation (fig. 247 A) the egg has a 

 nearly spherical form, and is constituted of a single layer of 

 columnar cells enclosing a small segmentation cavity. The 

 lower pole is slightly thickened, and the egg rotates by means of 

 fine cilia. 



An invagination now makes its appearance at the lower 

 pole (fig. 247 B), and simultaneously there become budded off 

 from tJie cells undergoing the invagination amoeboid cells, which 



1 The following classification of the Echinodermata is employed in this chapter. 



I. Holothuroidea. IV. Echinoidea. 



II. Asteroidea. V. Crinoidea. 



III. Ophiuroidea. 



