- ichthyoplankton
- incisor process
- incisure
- indicator species
- Infraciliature
- Infralaminal
- Ink sac
- Inner hump
- Inner organic lining
- Inter-rhopalar
- Intermediate canal
- Intermediate cirrus
- Internal spicule
- Interradial
- Interradius
- interramal
- interramal cirrus
- Interrhopalar canal
- interstitial
- intestinal diverticula
- intestine
- Intracapsular cytoplasm
- Intracapsulum
- Intracircular canal network
- iridescent
- Iridophores
- Ischium