Structure and function of the prehensile tentilla of <Emphasis Type="Italic">Euplokamis</Emphasis> (Ctenophora, Cydippida)

Euplokamis has coiled tentilla on its tentacles, which can be discharged, flicking out at high velocity, when triggered by contact with prey. The tentillum adheres to prey by means of numerous colloblasts. Discharge, which takes 40-60 ms, is accomplished by contraction of striated muscles, found only in this genus among the Ctenophora. Restoration of the coiled state is attributable to passive, elastic components of the mesogloea. Rows of "boxes" (fluid-filled compartments) along the sides of the tentillum appear to stiften the structure so that it does not collapse, kink or buckle during discharge. Smooth muscle fibres present in the tentillum may help pull the tentillum tight after prey have been captured. In addition to the rapid discharge response, the tentillum can perform slower, spontaneous, rhythmic movements which, it is suggested, resemble the wriggling of a planktonic worm, enabling the tentillum to function as a lure. These movements appear to be executed by contraction of two sets of myofilament-packed cells which differ in several important respects from conventional smooth muscle. They belong to a novel and distinct cytological subset ("inner-ring cells"), other members of which are packed with microtubules and seem to be involved in secondary structuring of the collagenous component of the mesogloea. Study of tentilla in different stages of development shows that the striated muscle fibres, originally nucleated, become enucleate as they differentiate and that the colloblasts form in association with accessory cells, as proposed by K.C. Schneider and G. Benwitz. The refractive granules which adhere to the outside of all mature colloblasts derive from these accessory cells. The colloblast nucleus undergoes changes during development suggestive of progressive loss of its role in transcription and protein synthesis, but it remains intact, contrary to statements in the literature. The tentillum of Euplokamis can be regarded as a true food-capturing organ and it is probably the most highly developed organ in the phylum.