THE SPOROZOA I 49 



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tioned the peristaltic contraction of certain Gregarinida, or the ener- 

 getic jerking motion sometimes observed in the same forms. None 

 of these movements, however, brings about a regular translation from 

 place to place, and the Sporozoa are regarded as the most sluggish 

 of the Protozoa. Food-seeking, which in free-living animals is the 

 main occasion for locomotion, is here unnecessary ; for the adult 

 animals, placed in the chyle of the host, or in the spaces between 

 cells and tissues, or in the cells themselves, have little occasion for 

 movement, save that which, in young forms, is necessary to reach the 

 host, to maintain their positions, and to prevent displacement. There 

 is, however, in certain forms of Gregarinida, a peculiar gliding motion 

 on the part of the adult organism. This is accomplished without 

 apparent exertion of any kind by the animal, and for a long time was 

 a puzzle to students of the group. Schewiakoff ('94) offered an 

 explanation, based upon actual observation and experiment, and 

 although very improbable at first sight, it is the only one thus far that 

 fits the case (Fig. 82). These observations have been confirmed re- 

 cently by Siedlecki ('00), who accepts Schewiakoff's interpretation, 

 while Lauterborn also gives a similar interpretation of the movement 

 in diatoms. According to Schewiakoff, the forward, gliding motion 

 is the result of the active secretion of the gelatinous substance from 

 the ectoplasm, which accumulates below the membrane to form a 

 gelatinous layer. The membrane of the cell, as described above, is 

 marked externally by clear longitudinal grooves, and the gelatinous 

 substance after filling these grooves, instead of spreading over the 

 surface of the membrane, flows down and backward in the grooves 

 to the posterior end of the body, where the secretion from different 

 furrows unites to form larger currents, and these, in turn, form still 

 larger streams, which, like a spider's web, solidify upon leaving the 

 body (D). Thus, a solid cylinder is formed behind the animal, the pos- 

 terior end of which fits into the basin-like depression like a cast in its 

 mould. The addition of new jelly by active secretion in the ectoplasm, 

 and the resistance of the solidified portion, causes a forward move- 

 ment of the animal. The movement, Schewiakoff further observes, 

 is only periodic, for the flowing of the jelly is more rapid than the 

 secretion — a fact which explains the occasional absence of the external 

 gelatinous layer. 



E. Reproduction 



The most characteristic phenomena connected with the Sporozoa 

 are those of reproduction and development. The many methods 

 occurring in the other forms of Protozoa are here limited to spore- 

 formation, although Labbe describes rather questionable simple divi- 



