CHAPTER XX. 



ECHINODEEMATA 1 . 



THE development of the Echinodermata naturally falls into two 

 sections : 



(1) 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 the 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 imagination now makes its appearance at the lower pole 

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

 cells undergoing the imagination amoeboid cells, which eventually 

 form the muscular system and the connective tissue. These cells very 

 probably have a bilaterally symmetrical origin. This stage repre- 

 sents the gastrula stage which is common to all Echinoderms. The 

 invaginated sack is the archenteron. As it grows larger one side of 



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



I. Hclothuroidea. IV. Echinoidea. 



II. Asteioilca. v. Crinoidea. 



in. Ophiuroidea. 



