Aplysia is a genus of sea slugs, commonly known as sea hares. They are marine mollusks with soft bodies and are often found in shallow waters. Aplysia are known for their distinctive large size and unique appearance.

The scientific name for the California sea hare is Aplysia californica.

Aplysia californica

The California Sea Hare, or Aplysia californica has gills inside its body. This is how it extracts oxygen from the water.

I believe varicosity in the context of your question refers to the enlarged and tortuous vein, artery, or lymphatic vessel in the area of the synapse. Therefore in the aplysia synaptic junctions between the sensory and motor neurons experience long term facilition in response to 5-HT and long term depression in response to RMRFamide. Research has shown macromolecules synthesis is needed for expression of the long lasting structural changes in the sensory cells and that this synthesis is porportional with long term functional modulation of sensorimotor synapses.