. Bulletin of the British Museum (Natural History). . Fig, 4. Hypolytus peregrinus Murbach, a solitary Corymorphine witli moniliform tentacles in both oral and aboral whorls (after Murbach, iSgg). Fig. 5. Euphysa mirata Forbes, a solitary' Corymorphine in which the oral whorl is capitate and the aboral whorl is moniliform ; hydranth of a young polyp, with tentacles fully extended (after Rees, 1937) Tricyclusidae, the Acaulidae, the Myriothelidae and in all the colonial capitate hydroids. As a basic type of tentacle it becomes duplicated on the body of the hydranth in the inter-whorl area in a
. Bulletin of the British Museum (Natural History). . Fig, 4. Hypolytus peregrinus Murbach, a solitary Corymorphine witli moniliform tentacles in both oral and aboral whorls (after Murbach, iSgg). Fig. 5. Euphysa mirata Forbes, a solitary' Corymorphine in which the oral whorl is capitate and the aboral whorl is moniliform ; hydranth of a young polyp, with tentacles fully extended (after Rees, 1937) Tricyclusidae, the Acaulidae, the Myriothelidae and in all the colonial capitate hydroids. As a basic type of tentacle it becomes duplicated on the body of the hydranth in the inter-whorl area in a large number of families. To return to the moniliform tentacle it appears that the aboral tentacles of Euphysa and Hypolytus hydroids have been retained in their primitive form only because the feeding habits of the hydroids favour the retention of the very long extensile fishing tentacle of the medusa. In these hydroids the tentacles are extended radially over the soft mud to trap any organism creeping over them. With the adoption of firmer substrata, the tentacles lost their need to be very extensile, this permitting a scattering of nematocyst armature and the evolution of a stouter, more rigid, and less contractile tentacle. This, the filiform type, is the aboral tentacle we have in Corymorpha nutans, the Tubularians, the Halocordylidae (Pennariidae) and the Acaulidae, and in vestigial form in the Corynidae. The moniliform arrangement still persists in aberrant survivals like the colonial hydroid Asyncoryne ryniensis Warren (Text-fig. 3) in which the aboral moniliform tentacles have become scattered over the body of the hydranth perhaps as a result of the lengthening of the body of the hydranth itself. In the solitary hydroid, Tricyclusa singularis, they persist only in a very imperfect form (Text-fig. 6). Both these forms could have arisen along independent lines from an Euphysa-like ancestor (see p. 514). Euphysa thus represents, as regards the hydranth, a basic type from
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