THE PTYCHOCYST, A MAJOR NEW CATEGORY OF CNIDA USED IN TUBE CONSTRUCTION BY A CERIANTHID ANEMONE

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Abstract
Light, transmission electron and scanning electron microscopy of a cerianthid anemone (Ceriantheopsis americanus) from Florida [USA] showed that the tube in which the animal lives is composed almost exclusively of a new cnida, the ptychocyst. As opposed to the threads of all nematocysts and spirocysts described to date, the ptychocyst thread is not helically folded in the undischarged state. Instead, it forms a variable number of stacked pleats in circumference (from 5-11 in this material) and no pleats in length. The discharged threads may be quite long and are non-isodiametric. The everted thread has a number of fine ridges running along its length which represent the folds by which the undischarged thread is compressed in diameter. The thread tip is closed and the entire thread is unarmed, lacking both spines and hollow tubules. The capsules are bilaterally compressed and are often large (up to 75 .mu.m long and 36 .mu.m wide) with perhaps 2 distinct size classes. The completely unique method of thread folding and pleating in both length and circumference indicates that ptychocysts are significantly different from either nematocysts or spirocysts, both of which have helically folded and pleated threads. Ptychocysts were included in the classification of coelenterate cnidae as a 3rd major category, equal in rank to both nematocysts and spirocysts.

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