IQ2 THE PROTOZOA 



D. ENCYSTMENT 



The phenomenon of encystment may be seen in the Ciliata as in all 

 other groups of the Protozoa. It occurs when the animal is in danger 

 of drying, in some cases before division, in others, for the purpose 

 of digesting a full meal. The cilia are drawn in, the mouth and peri- 

 stome disappear, the contained body-granules are voided, and a gelati- 

 nous secretion is poured out from the ectoplasm. The secretion 

 soon hardens, becoming chitinous. The vacuole continues to pulsate 

 for some time, and the secretion forms a liquid layer about the animal 

 under the cyst. The cysts are variously diversified with spines and 

 processes of different kinds, and are occasionally multiple, the spaces 

 between the cysts being filled with water (Fig. 17, B, C, F, p. 47). 



E. REPRODUCTION 



Reproduction among the Ciliata takes place almost exclusively by 

 simple division or fission. It is practically the same for all forms, the 

 variations being of minor importance. The nuclei first divide, new 

 mouth parts are developed in the posterior half, and then the cell 

 divides. The first indication of division in Stentor, for example, is a 

 rift in one side of the animal below the adoral zone. This rift rapidly 

 develops motile organs (membranelles), and acquires the full length 

 of the lower daughter-individual. The nuclei in the meantime divide, 

 and the original animal draws out, leaving a slender foot for the upper 

 or anterior cell, and a swollen portion for the pharyngeal region of the 

 second individual. The new adoral zone is completely formed before 

 actual division; the steps in the process are shown in the accompany- 

 ing diagram from Johnson ('93) (Fig. 106). The new contractile vacu- 

 ole, according to him, arises de novo in .S. coerulens, and by a dilatation 

 of the longitudinal canal in S- rceselii. The new vacuole thus formed 

 remains in connection with the longitudinal canal, the upper part of 

 which becomes drawn out with the torsion of the adoral zone to form 

 the much-discussed ring-canal discovered by Lachmann. 



In some forms, as in Spirochona, division simulates budding, un- 

 equal division giving rise to mother- and daughter-cells. 



When division takes place within the cyst, the various mouth parts 

 may or may not first be absorbed, but in all cases the vacuole still 

 continues to pulsate. Here, as a rule, division is double, resulting in 

 broods of four which escape as embryos, and gradually grow into the 

 parent form. This condition closely simulates spore-formation, which 

 results when, as in Opalina, the number of divisions within the cyst 

 reaches three, four, five, or six. 



Nowhere among the Protozoa has the process of conjugation been 

 so thoroughly studied in connection with the life-history of the organ- 



