72 SQUIRTS, POLYPS, AND JELLY-FISHES. 
size to the dimensions of small polyps, and each 
individual polyp secreting for itself an outer cover- 
ing of lime and an inner series of lime partitions 
(septa) corresponding to the fleshy ones that have 
been described, you have before you the struc- 
ture of a true coral, such as we find among the 
great reefs of tropical waters. A single species 
(Astrangia astreformis), which forms encrusting 
colonies on stone or shell from one to three inches 
across, 1s found on our northern coast, and has been 
occasionally picked up on the New Jersey shores, 
