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CONNECTICUT GEOL. AND NAT. HIST. SURVEY. 



[Bull. 



covered with vibrating cilia, and the embryo swims round and 

 round within the egg membrane. This membrane soon bursts, 

 and the free-swimming blastula rises from the sea bottom to 

 swim at the surface of the water. 



Fig. 16. Free-swimming larva (pluteus) 

 of the purple sea-urchin, Arbacia punctulata. 

 The deeply shaded portions represent the cal- 

 careous skeletal rods in the four pairs of 

 The primitive mouth, stomach, and 



arms. 



intestine occupy the central portions of the 

 body. (After Weysse.) 



After an infolding of the cells on one side to form a mouth 

 and a primitive alimentary canal, a complicated series of changes 

 in the embryo occurs. It finally assumes the shape of a helmet, 

 with long projecting arms, and bands of very long cilia. It now 

 swims freely about, usually in company with millions of others, 

 and begins to feed upon the minute organisms living at the 

 surface of the sea. The embryo, or larva, is now bilaterally 



