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.
As to the form of these circlets we have no clear knowledge.
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.