The cup, in its simplest form, consists of two circlets of five plates.
In Distomum acanthocephalum the cuticle forms circlets of large and small hooks at the anterior end, somewhat as in Cestodes.
In its present shape, dating substantially from the Renaissance, it is a peaked head-covering not unlike a closed mitre, round which are placed one above the other three circlets or open iCYzc=4- i FIG.
In Yoldia and Nucula proxima the test consists of five rows of flattened cells, the three median rows bearing circlets of long cilia.
They have two circlets of tentacles, a labial and a marginal, and there is only one ciliated groove in the stomodaeum, which appears to be the sulculus.
This crinoid has only two circlets of plates in the cup, but the cup analysed in the adjoining diagram has in addition infrabasals and a centrale C.
In addition to these three or four circlets of plates, two other elements were once supposed essential to the ideal crinoid: the dorso-central and the oro-central.
- Blastoidea in which the thecal plates have assumed a definite number and position in 3 circlets, as follows: 3 basals, 2 large and I small; 5 radials, often forkshaped, forming a closed circlet; 5 deltoids, interradial in position, supported on the shoulders or processes of the radials, and often surrounding the peristome with their oral ends.
As to the form of these circlets we have no clear knowledge.