From ZooTerms (Dictionary of Invertebrate Zoology)
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collarette (noun; Latin diminutive collare, band for the neck): 1. (Chaetognatha) Thickened distended cells in the neck region that in some species extend posteriorly along the trunk for some distance; in one instance as far as the lateral fins.

2. (NEMATA) Either anterior or posterior cuticular extensions forming an annular ring in the neck region.