The plates in the anterior paired ambulacra (areas II and IV) averaged 25.5% larger, whereas the plates in the posterior paired ambulacra (areas I and V) averaged 46.5% larger (fig.
parma, as well as the ambulacra. The interambulacra become discontinuous in a gradual sequence, from posterior to anterior (fig.
The suture between the plates in ambulacra I and V (the I-V suture) is typically the longest.
There is little variation in relative plate size, and the variation that does exist is not strongly correlated with the longitudinal position of the ambulacra.
In other northern Pacific sand dollars, the anterior and posterior ambulacra (and interambulacra) develop at different rates.
It assumes that a heterochronic change in the Echinarachnius pattern could simultaneously affect many aspects of test morphology, including the apical system, peristome, periproct, ambulacra, and interambulacra.