THE MAIN LINES OF ANIMAL EVOLUTION 



record even into pre-Cambrian times, and the Heliozoa, which are less 

 adequately known as fossils. 



Ciliata and Suctoria. In contrast to the classes Flagellata, Sporozoa, 

 and Sarcodina, all of which are fairly clearly interrelated, the two remain- 

 ing classes of Protozoa show no evidence of relationship to the above 

 named classes. These are the classes Ciliata and Suctoria, which some 

 protozoologists would prefer to assign to a separate phylum. The ciliates 

 have a definite shape, maintained by a pellicle, as do some flagellates. 

 There is a definite anteroposterior axis, and the animal is symmetrical, 

 sometimes radially, sometimes bilaterally, and sometimes an irregular 

 deviant from one of these types of symmetry. Nutrition is holozoic, with 

 other minute organisms being ingested by a mouth. Cilia are commonly 

 arranged in specialized tracts which sweep a food-bearing current toward 

 the mouth. It is often said that the Protozoa are the simplest of animals. 

 While this may be debated, there is little doubt that ciliates are the most 

 complicated of cells. The cilia themselves may be arranged as coordinated 

 tracts, or they may be fused in sheets to form undulating membranes, or 

 they may be fused in tufts to form appendage-like cirri. Whatever the 

 arrangement of the cilia, their movements are coordinated by a compli- 

 cated network of fibrils, the neuromotor system, which exceeds in com- 

 plexity the simplest nervous systems of Metazoa. The trichocysts, small 

 bodies which can be discharged to produce filaments, underlie the pellicle. 

 Their function is uncertain, but defense and attachment while feeding 

 have been the principal alternatives suggested. The food vacuoles, formed 

 at the mouth, pass through the body by a regular route and leave at a 

 definite point. In some, it would scarcely be an exaggeration to say that 

 there is a digestive tube and an anus. There are two nuclei, a micronu- 

 cleus which is concerned with heredity and a macronucleus which is con- 

 cerned with metabolic functions. This led Kofoid to state that it would be 

 as logical to call a whale unicellular as to call a Paramecium unicellular. 

 Finally, the ciliates have developed a unique type of sexual reproduction 

 called conjugation. The details are quite complicated, but in substance it 

 amounts to this, that the maturation divisions result in a stationary nucleus 

 and a wandering nucleus in each of a pair of conjugants, and the wander- 

 ing nucleus of each fertilizes the stationary nucleus of the other. 



The most primitive order of ciliates, the Holotricha, is completely 

 clothed in cilia, and the animals are strong swimmers. But feeding by 

 means of currents seems to lead to a sessile mode of life, for specialization 

 in the ciliates has generally resulted in attachment to the substrate, re- 

 striction of the cilia to limited areas usually related to feeding, and spe- 

 cialization of the ciliary tracts to form membranelles, undulating mem- 

 branes, or similar structures. The well known Vorticella is a good exemplar 

 of such tendencies. In a single order of ciliates, the Hypotricha, speciali- 

 zation has been in the direction of increased efficiency of locomotion. In 

 this order, a band of typical cilia still creates a current for feeding 

 purposes, but locomotion is based upon the use of fused tufts of cilia, 

 the cirri, which function in a leg-like fashion. 



Finally, the class Suctoria is a small group which is undoubtedly related 



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