The "low side" is opposite this and is centered near ambulacrum
I (e.g., ocular plate I, between interambulacra I and 5).
Architectural position of the fasciole within the frontal ambulacrum. Synapomorphy: fasciole crossing the ambulacrum on plates 6 or 7.
This relative displacement is linked with the balance between the anterior and posterior parts of the test (manifested in the multiplication of the plates in ambulacrum III).
In regular echinoids, each ambulacrum
(or interambulacrum) closely resembles the other four, and thus the test displays pentamerous radial symmetry.
It is encased in the connective tissue and calcite ossicles of the ambulacrum
and is dorsoventrally flattened in cross section [ILLUSTRATION FOR FIGURE 9 OMITTED].