The rhynchocoel is a
circulatory system
fluid-filled cavity
primitive excretory system
proboscis

fluid-filled cavity

Explanation:

Rhynchocoel is a fluid-filled tubular cavity present in the organisms of the phylum Nemertea. This cavity lies above the intestines of these organisms in the lower body.

Nemertea is a phylum composed of worms that can be found in shallow water.

These worms have already been classified as a branch of the Flatworms (such as Planarians), with which they are often confused. But recent embryological and molecular evidence points to a phylogenetic relationship with mollusks and annelids. They remind the Flatworms to be non-segmented flat worms. But unlike these, they have a complete bowel with mouth and anus, and a closed vascular system. The article on Planarians can be accessed here.

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