40 THE SUB-KINGDOM PROTOZOA. 



terminates in the Tentaculifera, the first important divergence from the 

 preceding track is encountered on arriving at the newly instituted order of 

 the Radio-Flagellata. This small group, to which at present are referred 

 the four genera, Actinomonas, Spongocyclia, Spongasteriscus, and Euchitonia, 

 may be said to retain the same relationship with respect to the ordinary 

 Radiolaria as subsists between the Rhizo-Flagellata and various other 

 orders of the Pantostomata. In the form Actinomonas, figured and 

 described for the first time in this volume, PI. I. Figs. 7, 8, and 18, the 

 permanent possession of a terminal vibratile flagellum alone distinguishes 

 it from a stalked Heliozoidal Radiolarian such as Actinolophus in its naked 

 phase, and with which type it was presumed at first sight to be identical. 

 Inversely, it needs only the withdrawal of the radiating pseudopodia, with 

 the retention of the flagellum, to produce the Pantostomatous Flagellate 

 genus Oikomonas. The primary derivation of the entire Radiolarian order 

 from the Flagelliferous section of the Pantostomata is clearly indicated in 

 association with the embryonic conditions of its representatives, and all of 

 which, so far as at present known, exhibit a Pantostomatous Flagellate 

 structure. The direct metamorphosis of such a simple flagellate zooid 

 into 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 sub- 

 sections of the Heliozoa, Monocyttaria, and Polycyttaria, onward to the 

 Tentaculifera, appears at first sight to be somewhat obscure. As shown, 

 however, in the chapter devoted specially to their description, the Tenta- 

 culifera, as now known, form among themselves two natural subordinate 

 groups or orders : the one, that of the Suctoria, being distinguished by the 

 sucker-like form and function of the radiating tentacles ; while in the other, 

 that of the Actinaria, these appendages closely resemble ordinary pseudo- 

 podia, being simply adhesive, and in some instances, e. g. Ephelota, invertile. 

 The transition from the Radiolaria to the Tentaculifera is apparently 

 accomplished through this last-named group, such types as Zooteira 

 and Actinolophus on the one hand, and Ephelota and ActinocyatJnis 

 on the other, representing the most conspicuous connecting forms. The 

 Tentaculifera, in their highest phase of development, exhibit several note- 

 worthy peculiarities. The embryos do not, as with most Radiolaria, take a 

 flagelliferous or monadiform contour, but are, while mouthless, more or less 

 thickly ciliate. The ciliation in the different genera and species, moreover, 

 varies considerably, the several more important deviations in this respect 

 exhibiting a remarkable conformity with the three types of ciliation that 

 characterize the three leading orders of the ordinary Ciliata, as distinguished 

 by the respective titles of the Holotricha, Hypotricha, and Peritricha. On 

 account of this last-named circumstance, it may be reasonably inferred that 

 some genetic relationship subsists between the two sections of the Poly- 

 stomata and Eustomata, this probable affinity being indicated in the 



