Club-shaped hydranths with numerous tentacles, generally scattered irregularly, sometimes with a spiral arrangement, or in whorls (" verticillate ").
On the other hand, the ejaculatory duct with its verticillate sac in the male of Cypris and other genera is a feature scarcely less remarkable.
The same occurs in the verticillate arrangement, the leaves of each whorl rarely being super- posed on those of the whorl next it, but usually alterna ting so that each leaf in a whorl occupies the space be tween two leaves of the whorl next to it.
These causes of change in phyllotaxis are also well exemplified in the alteration of an opposite or verticillate arrangement to an alternate, and vice versa; thus the effect of interruption of growth, in causing alternate leaves to become opposite and verticillate, can be distinctly shown in Rhododendron ponticum.
In monocotyledonous plants there is only one seed-leaf or cotyledon, and hence the arrangement is at first alternate; and it generally continues so more or less, rarely being verticillate.