PHYLUM V. CILIATA 



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Two wholly distinct nuclei exist in the Heterokaryota, a mega- or 

 macro-nucleus and a micronucleus. They are not merely different 

 in size, form, structure, and appearance, but also in function; for 

 the macronucleus is somatic and trophic in its function, while the 

 micronucleus is purely sexual. They are not comparable to the 

 tropho- and kineto-nuclei of the trypanosomes. 



The macronucleus is generally well marked, but breaks down 

 into granules before or after conjugation. It consists of an achro- 

 matic portion, with a chromatic portion in the form of a close- 

 meshed network of fibrils, but whether there is a definite surrounding 

 membrane or not is doubtful. 



The micronucleus, when at rest, is a minute irregular granule of 

 chromatin lying in the centre of a perfectly clear achromatic area. 

 Probably there is only one in each animal, and the appearance of 

 two or more is due to reproductive phases which are just finishing. 



The Ciliata are characterized by the presence of protoplasmic pro- 

 cesses projecting all over the body. These are fine, short whips, 

 called cilia, which in places are transformed into thick processes 

 called cirri or flat membranes in certain species. The cilia are pro- 

 cesses of the pellicle, but they appear to be supported by a thread 

 of specialized ectoplasm. The short, fine cilia are for motion, and 

 the long, motionless cirri for the purpose of entangling food-particles. 

 The membranes are supposed to be formed of fused cilia. Tricho- 

 cysts exist in the Holotricha, and an excretory organ has recently 

 been described by Metcalfe in certain species of Opalina, parasitic 

 in frogs, while other Ciliata have a contractile vacuole. 



Reproduction may take place asexually by (i) transverse or 

 longitudinal division; (2) gemmation, simple or miultiple; (3) en- 

 cystment and spore-formation, or sexually hy conjugation. 



1. Fission.' — There is no morphological distinction between longi- 

 tudinal and transverse fission, in which the following changes take 

 place: (i) A second mouth is formed; then comes (2) enlargement 

 and division of the micronucleus; followed by (3) enlargement and 

 division of the macronucleus; and, finally, (4) division of the 

 cytoplasm. 



2. Encystment and Spore-Forma^tion. — ^The animal encysts and 

 breaks up into a number of small individuals. 



3. Conjugation.- — Conjugation has been best described by Calkins 

 and CauUery iw Par anicccium aurelia. Two conjugating cells about 

 the same size are placed so that the mouths are directly opposed. 

 The micronucleus swells, its chromatin becomes granular, elongated, 

 crescentic, fusiform, and finally forms two nuclei, each of which at 

 once divides into two. Of these, two degenerate, while the other two 

 divide to form a migratory, or male, and a stationary, or female, pro- 

 nucleus. The male interchange and fuse with the opposite female 

 nuclei, and then the organisms separate. The macronucleus frag- 

 ments and disappears, and the synkaryon breaks up into eight 

 micronuclei. - 



Four of these swell, and are changed into macronuclei; so that 



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