250 ORDER FLAGELLATA-PANTOSTOMATA. 



GENUS I. OIKOMONAS, S. K. 

 (Greek, eoika, resembling ; monas.) 



Animalcules exceedingly minute, plastic and unstable in form, ovate, 

 globular, or elongate, sometimes free-swimming and sometimes attached by 

 a temporarily developed thread-like prolongation of the posterior extremity 

 of the body ; flagellum single, anteriorly located, subservient when swimming 

 to the purpose of locomotion and in the attached condition to bringing food- 

 particles within reach, these incepted at any portion of the periphery ; con- 

 tractile vesicle and endoplast usually conspicuous. 



HAB. Fresh and salt water, abundant in infusions. 



This new generic title is introduced for the reception of all those uniflagellate 

 species that correspond precisely in their free-swimming state .with those of the preced- 

 ing genus Monas, but which possess in addition the faculty of attaching themselves at 

 will to foreign bodies through the medium of a thread-like extension of the sarcode 

 of the posterior end of the body. Preferring again to pursue a nomadic life, this 

 extemporized pedicle is withdrawn into the substance of the parenchyma, and the 

 animalcules swim away under conditions and appearances identical with those pre- 

 sented during their previous wandering state. As a necessary consequence, an 

 acquaintanceship of some duration is in most instances absolutely requisite for the 

 precise determination as to which of the two genera, Monas or Oikomonas, certain 

 animalcules should be referred. In their more typical fixed or stalked condition 

 the identification of the representatives of the last-named genus presents no difficulty ; 

 but the same zooid, as hereafter shown, sometimes exhibits in its nomadic state an 

 aspect so entirely divergent from the fixed one, that unless the passage from the one 

 to the other has been actually witnessed, their specific relationship would not so much 

 as be suspected. With the typical form Oikomonas mutabilis, here introduced, has 

 naturally to be included the Monas termo of Professor H. James-Clark, recently 

 demonstrated by that authority * to possess a stalked as well as a free-swimming 

 condition. The possession of a single flagellum only instead of one long and one or 

 two shorter ones, serves to distinguish Oikomonas respectively from the two genera 

 Physomonas and Spumella. 



Oikomonas mutabilis, S. K. PL. XIII. FIGS. 55-64. 



Body plastic and variable in form in the attached condition symmetri- 

 cally ovate, pyriform, or subspherical, seated on a slender pedicle about 

 equal to the body in length in the free-swimming condition changing 

 from spherical or ovate to an elongate contour, about three times as long as 

 broad, with a rounded and wider posterior extremity, a slightly constricted 

 central portion, and a bluntly pointed and somewhat truncate anterior 

 border ; flagellum long and slender, inserted at the apical extremity, when 

 swimming held arcuately and apparently rigidly in advance ; parenchyma 

 colourless, more or less granular, enclosing anteriorly a spherical endoplast, 

 and posteriorly two contractile vesicles. Dimensions of subspherical 

 attached body 1-1500" ; length of elongate free-swimming zooids 1-750". 



HAB. Vegetable infusions in fresh water ; gregarious ; motion in the 

 water straight and even. 



* 'Memoirs Boston Soc. Nat. Hist.,' vol. i., part iii., 1868. 



