454 



THE PROTOZOA 



suppressed, and has been replaced by the peculiar type of syngamy charac- 

 teristic of the group. 



The question of the exact systematic position of Opalina cannot be decided 

 until more is known of the life-cycles of other parasitic Ciliata ; but at present 

 there do not seem to be any cogent reasons for removing this genus from the 

 Ciliata. 



Affinities of the Ciliata. A typical ciliate, such as Paramecium, with its 

 even coat of fine cilia, its heterokaryote nuclear apparatus, and its peculiar 

 type of syngamy with partial karyogamy, stands apart and apparently 

 isolated from the typical members of other classes of the Protozoa. Never- 

 theless, even within the limits of the class Ciliata, 

 examples are to be found in which the heterokaryote 

 condition is not developed, or only appears prior 

 to syngamy in the form of a separation of generative 

 from vegetative chromatin (Trachelocerca, Opalina), 

 and in which the syngamy takes the form of total 

 karyogamy between minute gametes, swarm-spores 

 (Opalina). Such cases, while they minimize the gap 

 between Ciliata and other Protozoa, do not bring the 

 ciliates nearer to any particular class, since a similar 

 type of syngamy and of preparations for it may occur 

 either in Sarcodina or Mastigophora. 



As the most distinctive feature of the Ciliata there 



remains that which is 

 implied in the name- 

 that is to say, the posses- 

 sion of cilia. As has been, 

 pointed out above, how- 

 ever, a cilium is similar 

 to a flagellum in every 

 essential point of structure 

 and function. There can 

 be no doubt that the 

 ciliary covering represents 

 a large number of flagella 

 specialized in respect to 

 size, number, arrange- 

 ment, and co-ordination. 

 It has been mentioned 

 above that some flagel- 

 lates, such as the Tricho- 

 nymphidce and allied 

 forms, are regarded by 

 some authorities as transi- 

 tional from the Flagellata 

 to the Ciliata. It is per- 

 haps improbable, how- 

 ever, that the transition 

 from the one group to 



e.v. 



FIG. 189. A, Monomastix ciliatus : ft., flagellum ; 

 o, mouth ; N, macronucleus ; n, micronucleus ; 

 c.v., contractile vacuole ; a., anus, near which opens 

 the efferent canal of the contractile vacuole. After 

 Roux, magnified 1,000. B, Maupasia paradoxa : 

 as., oesophagus ; other letters as in A. After 

 Schewiakoff, magnified 1,300. 



the other should have been 

 through endoparasitic forms ; and it is on the whole more likely that free- 

 living forms, such as the holomastigote genus Multicilia, are the nearest 

 representatives of the earlier ancestral forms of the Ciliata. 



Two interesting forms have been described which combine in some respects 

 the characters of both Flagellata and Ciliata. 



Maupasia paradoxa (Fig. 189, B) is described by its discoverer, Schewiakoff 

 (863), as having the body metabolic, with cilia in the anterior part of the 

 body, and the remainder covered with long flagella. At the hinder end of the 

 body is a longer flagellum implanted close beside the aperture of the efferent 

 duct of the contractile vacuole. The mouth-opening, on the ventral side of 



