60 BRANCH ECHINODERMATA 



The nervous system is upon the same plan as that of the 

 starfish. 



Multiplication. The eggs are laid in the water and fertilized 

 by the tadpole-like sperm cells. Some forms have a marsu- 

 pium, or brood-pouch, in which the eggs are hatched. 



Development. After fertilization, segmentation of the egg 

 takes place until the bilaterally symmetric young " pluteus," 

 which is very unlike the adult, appears. It is free swimming 

 and lives on minute organisms it can procure in the water. As 

 it develops it takes on the radiate or pentameral plan of its 

 branch. The " sand dollars " so common on both the Pacific 

 and the Atlantic coasts are flat sea-urchins with short spines. 



Geologic Distribution. A primitive type of sea-urchin ap- 

 peared in the Ordovician period. 1 



CLASS IV. HOLOTHUROIDEA 



Holothurians are free, and a close examination reveals the 

 pentameral plan of the branch, although they are more or less 

 bilaterally symmetric. 



Fig. 46. Cucuma'ria f rondo' sa, side view. Note tentacles and rows of 

 feet. (Clark, in Bulletin 550, U. S. F. C., 1902.) 



The shape (Fig. 46) is much like that of the garden cucumber 

 in our common varieties, but some are long and slender and 

 Scott's "Geology," p. 381. 



