other hiding-place and swims freely about, to be- 

 come the frequent prey of the roving sea-robin or 

 of lurking flat-fishes. 



In keeping with the specialization exhibited in 

 the rest of its structure, Nereis has a blood-vascu- 

 lar system. The blood is pumped through the body, 

 not by a heart such as that organ is commonly con- 

 ceived, but by a great blood-vessel lying along the 

 back, which periodically contracts with peristaltic, 

 or wave-like, movements along its entire length. 

 Such oxygen as it derives must be absorbed 

 through its gills; as a consequence these are con- 

 tinuously in motion, fanning the water, as it were, 

 for fresh supplies. 



The propensity of free-swimming polychaetes 

 for appearing in the open only after dark, is re- 

 flected in their habits of spawning. The eggs are 

 freed in midsummer, always in the night, and usu- 

 ally immediately following sundown. From the 

 manner in which many observers write, it would 

 seem that this habit of spawning at night-time is a 

 puzzling peculiarity. Just why this should seem so 

 singular, I am at a loss to understand. The matter 

 is really simple. Indeed, it would be odd were the 

 worm to spawn at any other period. It obviously 



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