ZOOLOGICAL AFFINITIES. 105 



forming the peripheral series are simply prehensile, and catch and convey 

 food to the central ones. 



Should a type be discovered possessing, in addition to the peripheral 

 or prehensile series, only one central ingestive tentacle, in place of the three 

 or four that more usually obtain, and which would consequently occupy 

 that position in relation to Hemiophrya that is maintained by Rhyncheta 

 with respect to Podophrya, there would be undoubtedly presented an 

 organism directly comparable with an ordinary gustatory Hydroid Polypite. 

 The homoplastic correlation of the Tentaculifera with the Hydrozoa can 

 be still further extended. In the compound type Dendrosoma radians, 

 for example, we meet for the first time, and, as far as yet known, with the 

 only existing instance among the Protozoa in which is produced a decum- 

 bent creeping stolon and an erect branching stalk, essentially analogous 

 with the so-called creeping " hydrorhiza " and erect " hydrocaulus," which 

 form the common " ccenosarc " of the ordinary Hydroid Zoophytes. The 

 phenomena of development in this group yield yet another highly 

 suggestive co-ordination in a similar direction. As previously stated, the 

 larval condition of the Ccelenterata generally, including the Hydrozoa, is 

 a free-swimming, mouthless, ciliated sac, named a Planula, with which 

 certain of the Opalinidre have been indirectly compared. It is a note- 

 worthy fact that the larval condition of the Tentaculifera is also in all 

 cases a free-swimming and mouthless ciliated sac entirely different from 

 the parent organism, whose cilia become similarly obliterated on the 

 assumption of the normal sedentary state. 



In addition to the foregoing enumeration of the several apparent homo- 

 plastic affinities here suggested as subsisting between the various sections of 

 the Infusoria with relation to certain Metazoa, or to the more simple 

 elements of the same, the morphologic relationship or position of these 

 several groups or sections with respect to each other remains to be 

 summarized. The main lines or phyla of evolution, from the lowest and 

 most simplified factors in the series as represented by such types as 

 Mastigamceba or Monas, by diversely branching and frequently intercalat- 

 ing tracks, up to the highest members of the class, has been already dis- 

 cussed at length, and diagrammatically illustrated in a preceding chapter. 

 The more intricate question, however, as to which special group or sub- 

 order of the Infusoria generally, or of the Ciliata and Tentaculifera in 

 particular, claims precedence as exhibiting the most highly differentiated 

 structural type, has yet to be decided. More usually this most advanced 

 position has been conceded to the Peritrichous sub-order of the Ciliata, 

 including chiefly the Vorticellidae, whose higher structural plan is held 

 to be manifested through the reduction and concentration of the ciliary 

 appendages so as to form a simple circular or spiral adoral wreath. Viewed 

 also with respect to the closely approximated positions of the oral and anal 

 apertures, and to the high organization of the pharyngeal apparatus, the 

 evidence would appear to be altogether in favour of the Peritricha. 



