Very small sea-urchins with an elongated or round, usually flattened test.
The mouth is located in the middle on the under side; the anus is also on the under side posterior to the mouth. No gills or gill-cuts at the peristomial edge of the test. There is a well developed dental apparatus (lantern).
The interior of the test is usually with a complicated system of supporting pillars or walls, connecting the upper and lower floor. The ambulacral plates are simple; on the upper side the ambulacra are usually widened (petaloid), forming a flower-like figure. The tubercles are perforate. Spines are quite small, forming a close almost velvet-like coat.
Tube-feet and pedicellariae
The tube-feet are small, usually many to ambulacral plate and often spreading also over the interambulacra. There are only three types of inconspicuous pedicellariae. Spaheridae are present, but placed in grooves in the test so as not to be externally visible.
This Order is richly developed in tropical seas, but in the North Sea only represented by the family Fibulariidae.
- Family Fibulariidae