1. An experimental analysis of ciliary metachronism was performed on ctenophores of two types: those with a continuation of the ciliated groove running between adjacent comb plates (lobates), and those in which the ciliated grooves end at the first plate of each comb row (cydippids and beroids).

2. The results showed that the comb plates of cydippids and beroids are co-ordinated by mechanical forces arising from the movement of the plates themselves. Only along the ciliated grooves and at their junctions with the comb rows is a neuroid conduction process likely in this group of ctenophores.

3. Ctenophores with an interplate ciliated groove, on the other hand, do not depend on mechanical interaction between active plates for co-ordination. Instead, the lobates use neuroid conduction at the junction between the interplate ciliated groove and the comb plate, at the junction between adjacent interplate ciliated grooves, and probably along the ciliated grooves and interplate ciliated grooves as well. Ciliary co-ordination may therefore be entirely neuroid in lobate ctenophores.

4. In both types of ctenophores the comb plates may be triggered to beat by depolarization of the comb-plate cells. Electiical excitation of a plate could be caused by mechanical deformation of its base arising from the motion of the preceding plate (in cydippids and beroids), or by a neuroid process at the interplate ciliated groove-comb plate junction (in lobates).

5. These findings resolve a long-standing controversy over the mechanism of ciliary co-ordination in ctenophores.

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