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

¹ Department of Biological Science, Florida State University, Tallahassee, Florida 32306

Light, transmission electron and scanning electron microscopy of a cerianthid anemone reveal that the protective tube with which the animal surrounds itself is composed almost entirely of discharged, interwoven cnidae of a heretofore undescribed type.

As opposed to the threads of all nematocysts and spirocysts described to date, the thread of the new cnida, to which has been given the name of ptychocyst, is not helically folded, forming a variable number of pleats in circumference (from 5 to 11 observed in the present study), and no pleats in length. The discharged threads are quite long (over 2 mm in some) and are non-isodiametric, tapering from a diameter of about 5 µm at the base to about 2 µm at the tip for an overall reduction in diameter of 2.5 times.

The everted thread has a number of fine ridges running along its length which can be seen using phase contrast microscopy and which represent the folds by which the thread is compressed in diameter. The thread tip is closed and the entire thread is unarmed, lacking both spines and hollow tubules. The undischarged and partially discharged threads are both basophilic and acidophilic. The capsules are bilaterally compressed and are often large (up to 75 µm long and 36 µm wide) with perhaps two distinct size classes.

The completely unique method of thread folding indicates that ptychocysts are significantly different from either nematocysts or spirocysts, both of which have helically folded and pleated threads. Ptychocysts have therefore been included in the classification of coelenterate cnidae as a third major category, equal in rank to both nematocysts and spirocysts.

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