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.
Spiders do not have muscles for extending the legs at these joints. Spiders, like most creatures, desiccate upon death, losing their waters, explaining why spider legs curl up upon death.
