1902.] NATURAL SCIENCES OF PHILADELPHIA. 5 



Sipunculiis, and allowing its abundant pink perivisceral fluid to 

 run into a glass dish, my attention was attracted by two white 

 flakes, of about an eighth of an inch in length, which were swim- 

 ming actively in the liquid. Their movement was like that of 

 some planarians, and seemed to depend on the undulation of their 

 lateral margins, which were plainly to be seen in a state of vibra- 

 tion. These white flakes turned out to be specimens of Monocystis 

 sipuncuU." 



It is probable, however, that the movements here seen were noth- 

 ing but the violent contortions which gregarines frequently show 

 when first removed from their native environment. Such contor- 

 tions might readily cause progression were the animals floating 

 freely in a fluid. Moreover, Lankester himself appears never to 

 have laid much stress on this single observation, for to my knowl- 

 edge it is not referred to again in any of his later contributions on 

 the Gregarinida. 



Frenzel (1891 p. 287 et seq.) suggested that the progression of 

 gregarines is due to a chemotactic afiinity between them and their 

 food. Such an explanation, however, is manifestly inadequate. 



Schewiakoff (1894), as the result of a painstaking study, came to 

 the conclusion that gregarines progress by means of the extrusion 

 of gelatinous fibres. These fibres are derived from a layer of sub- 

 stance which is deposited between the cuticle and the ectoplasm. 

 They pass out to the exterior through slit -like openings through the 

 cuticle which occur in the grooves between the longitudinal thick- 

 enings. Upon their emergence, they do not project radially from 

 the surface of the gregarine, but run backward until the posterior 

 end of the animal is reached. Somewhat hardened by the action 

 of the surroimding watery media, they then project backward and 

 free of the animal. This extrusion, which takes place over the 

 entire surface of the gregarine, results in the formation behind it 

 of a hollow cylinder, the walls of Avhich have by now acquired a 

 certain amount of rigidity. The posterior end of this cylinder, 

 impinging upon some resistant body, becomes fixed. The extrusion 

 continuing, the cylinder lengthens, and the gregarine is pushed 

 passively forward. 



Schewiakoff undertook his studies in the light of Lauterborn's 

 discovery that diatoms progress by means of the extrusion of gelati- 

 nous threads. There is a difference iu the progressive nioveraents 



