SEA-FANS AND SEA-PENS 153 
what firm crust. This, in dried specimens, breaks off readily. 
The colony branches profusely, or rather separates by fission, in 
one plane, the large and small branches making a network of 
fan-like shape and often of great fineness and intricacy. These 
fans attain a length of several feet and a corresponding breadth, 
and are abundant in semi-tropical as well as in more southern 
waters. In color they are red, yellow, brown, or purple. 
ORDER PENNATULACEA 
SEA-PENS 
These singular colonies resemble quill-feathers, such as are 
used for pens, and are named for this reason. The long, horny 
central axis is naked below, and is partly buried in the sand, but 
is not permanently attached. The upper portion of the axis has 
two rows of short, opposite, lateral branches; on the upper side 
of the branches the polyps live in separate inclosures. Spicules 
of lime are present in the substance of the branches, which gives 
them stability. Sea-pens are found at moderate depths and are 
widely distributed. They are highly colored and phosphorescent. 
