actinotrichium

from The Century Dictionary.

• noun One of the homogeneous hair-like fibers which represent the rays in the fin-folds of the embryos of fishes, and which subsequently fuse to form the membranous basis of the permanent rays of the adult fish.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology Any of a group of hairlike fibres in the finfolds of fish larvae that develop into fin rays