Peristome

The oral area of Hypsiclavus guensburgi from Sumrall (1996). A-E are the names of the distal ambulacra. Primary plates (colored grey on the anterior side) alternate with secondary plates (not colored). PO = primary oral plates marking the division of the A ambulacrum and the laterally shared ambulacra. LBP = lateral bifurcation plates that mark the division of the laterally shared ambulacra into the B and C, and D and E ambulacra.

The peristome, hydropore, and gonopore of discocystinids form a largely integrated structure covered by cover plates and a series of hydropore orals. The ambulacra radiate from the peristome and are lettered A - E clockwise starting with the anterior ambulacrum. The oral cover plates generally are similar to their ambulacral counterparts with primary and secondary plates. Four of the cover plates, however, are somewhat distinct from the others. The primary oral plates separate the anterior or A ambulacrum from the two lateral, or shared, ambulacra. Indeed the primitive condition for all Echinodermata is to have three ambulacra leaving the peristome rather than the five that is typically associated with this group. Five ambulacra are formed where the shared ambulacra split marked by the lateral bifurcation plates.

An interior view of the peristome of Hypsiclavus huntsvillensis. OF = oral frame plates, P = peristome, H = hydropore, G = gonopore.

In discocystinids the peristomal opening is known from the interior of several different specimens and is surrounded by five oral frame plates (centered along the ambulacra) that are thickened around the peristome forming a distinct lip. Where known, the peristome is uniformly bean-shaped often appearing narrowest centrally along the A oral frame plate. The A oral frame plate is symmetrical with its greatest with and thickness proximally. Distally, it is much like an ambulacral floor plate, but the food groove widens and becomes deeper towards the peristome. The B through E oral frame plates are asymmetrical in accordance with the other thecal openings they share and border a greater portion of the peristomal towards the A oral frame plate than towards the C or D oral frame plates. They share long sutures with the C and D oral frame plates respectively that correspond with the corners of the peristome.

The hydro-gonopore externally is a combined opening for the water vascular system (hydropore) and genital opening (gonopore). It is highly raised and positioned posterior-right of the peristomal opening. Anteriorly it is formed from three or more plates that border along foreshortened posterior oral cover plates. Posteriorly it is formed from three or more plates that are, in most formes, bordered by one to three rows in imbricate plates. The left margin is formed from a single plate. On the interior, the hydro-gonopore structure can clearly be seen to have two openings. The C oral frame plate shares half of the posterior margin of the peristome, the anterior margin of the gonopore, and the right margin of the hydropore. The posterior margin of the smaller, triangle-shaped gonopore is formed from a single plate, probably one of the posterior hydropore oral plates. The circular hydropore is bordered on the C and D oral frame plates.