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Abstract
Cnidaria and ctenophores, though of the ‘tissue grade of construction’, can form organs of considerable complexity, for example the prehensile tentilla of Euplokamis, the erupting nematocyst batteries of the siphonophores Stephanophyes and Nanomia and the complex eyes of cubomedu‐sae such as Carybdea and Tripedalia. The polypoid and medusoid members of siphonophore colonies are functionally equivalent to organs and may be considered as ‘zooid‐derived organs’. There is no reason to regard the lack of a single, dominant nerve centre as a factor constraining organ development in coelenterates.