Anatomy, apical system, clypeasteroid, Dendrasteridae, development, eccentricity, echinoid, evolution, periproct, peristome, sand dollar.
The periproct, or anal opening, is also located centrally, within the plates of the apical system.
In irregular echinoids, the periproct migrates from the apical system into the posterior interambulacrum.
Many irregular echinoids are anatomically polarized: the peristome lies near the anterior end of the test, whereas the periproct lies at the posterior end (fig.
In many holectypoids and clypeasteroids, the peristome and the periproct are both found on the oral surface of the test: the peristome typically lies near the center, whereas the periproct lies between the peristome and the posterior margin (fig.
This phenomenon may be termed "retrodisplacement," because the direction of periproct movement is opposite to the expected direction of periproct movement in irregular echinoids.
During the course of dendrasterid evolution, the periproct has shifted anteriorly in the manner described above, from the posterior margin onto the oral surface.
Together, these two papers provide a comprehensive model for the evolution of the characteristic features of the dendrasterid test, including the peristome, periproct, apical system, ambulacra, and interambulacra.
The retrodisplacement of the peristome and periproct are two of the effects predicted by the developmental model.