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Cymodocea. A pulpy stem, marked with rings in 
the lower part, and smooth in the upper. Cells fili- 
form, alternate or opposite. 
GENERA. 
BELONGING TO THE THIRD ORDER OF THE FIRST CLASS. 
Tibiana. Branches in zig-zags. Polypous openings, 
placed alternate, and on the sides, at the summits of 
the angles. 
Naisa. In fresh water. Polypi with each a single 
row of tentacula, generally ciliated. 
Tubularia. Tubular, simple or branched. A soli- 
tary polypus at the extremity of each branch. 
