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.