leech
蛭 centers on the leech, a blood-sucking worm. The meaning is specific and concrete, with no major abstract extensions.
蛭 combines 虫 (insect/worm) with 至, which likely contributes the sound. The character specifically denotes a leech.
A worm 虫 that arrives 至 to suck blood: that's a leech.
For シツ, imagine a leech sitting on your skin: sit -> シツ, and the leech is right there.
leech
Rhizophora mucronata (species of mangrove)
Hirudo nipponia (species of blood-sucking leech)
land leech (Haemadipsa zeylanica japonica)
Helobdella stagnalis (species of leech)
sheep liver fluke (Fasciola hepatica)
fascioliasis
mangrove (of family Rhizophoraceae)
grey mangrove (Avicennia marina); white mangrove
vermiculite
Potamogeton distinctus (species of pondweed)
Hirudo nipponia (species of blood-sucking leech)
black mangrove (Bruguiera gymnorrhiza)
Kandelia obovata (species of mangrove)
Whitmania pigra (species of leech)