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40 THE SUB-KINGDOM PROTOZOA.terminates in the Tentaculifera, the first important divergence from thepreceding track is encountered on arriving at the newly instituted order ofthe Radio-Flagellata. This small group, to which at present are referredthe four genera, Actinomonas, Spongocyclia, Spongasteriscus, and Euchitonia,may be said to retain the same relationship with respect to the ordinaryRadiolaria as subsists between the Rhizo-Flagellata and various otherorders of the Pantostomata. In the form Actinomonas, figured anddescribed for the first time in this volume, PI. I. Figs. 7, 8, and 18, thepermanent possession of a terminal vibratile flagellum alone distinguishesit from a stalked Heliozoidal Radiolarian such as Actinolophus in its nakedto be identical.phase, and with which typeit was presumed at first sightInversely, it needs only the withdrawal of the radiating pseudopodia, withthe retention of the flagellum, to produce the Pantostomatous Flagellategenus Oikomonas. The primary derivation of the entire Radiolarian orderfrom the Flagelliferous section of the Pantostomata is clearly indicated inassociation with the embryonic conditions of its representatives, and all ofwhich, so far as at present known, exhibit a Pantostomatous Flagellatestructure. The direct metamorphosis of such a simple flagellate zooidinto the Radiolarian type Actinophrys, as recently observed by the author,will be found recorded in the systematic description of the Radio-Flagellata,and is illustrated at PI. I. Figs. 9-11.The passage from the extensive group of the Radiolaria with its subsectionsof the Heliozoa, Monocyttaria, and Polycyttaria, onward to theTentaculifera, appears at first sight to be somewhat obscure. As shown,however, in the chapter devoted specially to their description, the Tentaculifera,as now known, form among themselves two natural subordinategroups or orders the :one, that of the Suctoria, being distinguished by thesucker-like form and function of the radiating tentacles ;while in the other,that of the Actinaria, these appendages closely resemble ordinary pseudopodia,being simply adhesive, and in some instances, e. g. Ephelota, invertile.The transition from the Radiolaria to the Tentaculifera isapparentlyaccomplished through this last-named group, such types as Zooteiraand Actinolophus on the one hand, and Ephelota and ActinocyatJnison the other, representing the most conspicuous connecting forms. TheTentaculifera, in their highest phase of development, exhibit several noteworthypeculiarities. The embryos do not, as with most Radiolaria, take aflagelliferous or monadiform contour, but are, while mouthless, more or lessthickly ciliate. The ciliation in the different genera and species, moreover,varies considerably, the several more important deviations in this respectexhibiting a remarkable conformity with the three types of ciliation thatcharacterize the three leading orders of the ordinary Ciliata, as distinguishedby the respective titles of the Holotricha, Hypotricha, and Peritricha. Onaccount of this last-named circumstance, it may be reasonably inferred thatsome genetic relationship subsists between the two sections of the Polystomataand Eustomata, this probable affinity being indicated in the