Interambulacral Plates

The oral surface of Hypsiclavus priesti. A = ambulacra, IA - interambulacra.

Interambulacral plates are the series of plates on the oral surface forming the thecal wall between the ambulacra. These plates are polygonal, fairly thick, and adjacent unlike homologous plates found in more primitive forms that are thin, scale-like, and highly imbricate.

A large interambulacral plate from the center of the interambulacrum of Spiraclavus nacoensis. Note the curved sutures, large size, and equitorial notch "E".

The sutures between interambulacral plates are unusual in that each plate has an equatorial notch. Typically, interambulacral plates are largest in the center of the interambulacral areas, except for CD where the anal pyramid is positioned. These larger plates often have curved sutures. Away from the interambulacral centers, the plates become slightly smaller and have straight sutures.

SEM photographs of a typical interambulacral plate of Spiraclavus nacoensis. Note the polygonal shape, the straight lateral sutures and the equitorial notch "E".

The interambulacral plates next to the ambulacra are typically smaller and have a long suture with the floor plates and develop lateral extensions on their interior surface that articulate with the lateral extensions of the floor plates.

SEM photographs of an interambulacral plate next to the ambulacrum (note that the interior surface "I" is towards the top and the exterior surface "E" is towards the bottom) of Spiraclavus nacoensis. L = lateral extensions that articulate to the lateral extensions of the floor plates.