Hickson, "Coelenterata and Ctenophora," Cambridge Natural History, i.
In most rotifers, on the contrary, the trochus is stronger than the cingulum, often lobed, and with some of its cilia aggregated into vibratile styles homologous with the combplates of Ctenophora (q.v.) and the membranelles of ciliate Infusoria (q.v.).
- Dorsal view of Coeloplana to illustrate the similarity between Ctenophora and Turbellaria (X 12).
Ctenophora possess two similar long branched tentacles, a ventral mouth and dorsal otolith.
The grounds for this view are the radial symmetry of several Polyclads and the supposed origin of gonads and excretory flame-cells from the walls of gut, the occurrence of nematocysts in Anonymus, one of the most radially constructed Polyclads, and lastly the presence of two peculiar animals Ctenoplana and Coeloplana, which suggests a transition from Ctenophora to Polyclads.
The Ctenophora are so aberrant in structure that it has been proposed to separate them from the Coelentera altogether: they are, however, theoretically deducible from an ancestor common to other Coelentera, but their extreme specialization precludes the idea of any close relationship with the rest.
Huxley's Actinozoa comprised the sea-anemones, corals and sea-pens, on the one hand, and the Ctenophora on the other.