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.
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