Where the sac is completely closed it generally contains no solid particles, but in a few Macrura a single otolith secreted by the walls of the sac is present.
The mouth is shown by the line surrounding the otolith (OT) in the centre.
The mouth is ventral, the otolith dorsal.
In Nausithoe a single large otolith is found: Natural size.
A single otolith is present as in the veliger embryos of Opisthobranchia.