ORAL APERTURE. 67 



Oral Aperture or Cytostome. 



It is necessarily only in connection with the members of the Eusto- 

 matous section of the Infusoria, including the three orders of the Flagellata- 

 Eustomata, Cilio-Flagellata, and Ciliata, that a true oral aperture or, as 

 Haeckel designates it in contradistinction to the oral opening of the multi- 

 cellular animals, a " cytostome " is met with, food-substances in the 

 remaining sections being incepted indifferently over the whole or a more 

 or less widely dispersed area of the peripheral surface. Where, as above 

 indicated, such a distinct oral aperture is represented, a considerable amount 

 of variation is found to subsist with relation to its contour and associated 

 structure. With the great majority, the oral aperture takes the form only 

 of a simple orifice, or of a tubular passage, through which direct inter- 

 communication is established between the surrounding medium and the 

 inner or deeper endoplasmic region of the animalcule's body. Most 

 frequently this oral aperture is permanently conspicuous, but not un- 

 frequently, as in the genera Dinomonas and Trichoda, it happens that this 

 structure is to be detected only at the moment when food is being swal- 

 lowed, its lateral walls at all other times closing so completely upon each 

 other as to leave no passage visible. As might be anticipated, when the 

 walls of the oral passage are loose and elastic, as in the above-named types, 

 this orifice is capable of great distension, the food mass devoured being 

 frequently but little inferior in size to the body of its captor. The first 

 manifestation of a complex organization in connection with the oral aper- 

 ture is indicated by a simple thickening or induration of the lining wall ; 

 this thickening in certain types, such as TracJielophylhim apiculatum, takes 

 the form of well-developed longitudinal rugae, while in others, such as 

 Chilomonas paraineciuin> this region has been lately shown by Biitschli to 

 be both longitudinally and transversely plicate. Among the majority of 

 the Dysteriadae, as also in association with many of the Prorodontidae, 

 a distinct corneous tube, that may be isolated from the surrounding body- 

 plasma, is substituted for the simple indurated oral passage characteristic 

 of the last-named series ; this structural type leads again directly to that 

 considerable assemblage of forms in which, in place of such a simple 

 corneous tube, a tubular fascicle of rod-like teeth or stylets is enclosed 

 within, but remains at the same time capable of protrusion at will beyond, 

 the oral fossa, and is employed by the animalcule for the purpose of 

 grasping and engulphing its accustomed prey. This special type of oral 

 armature is, so far as it is at present known, possessed only by the Ciliate 

 section of the Infusoria, occurring, however, in three out of the four 

 leading groups or orders of this division, as represented by the genera 

 Prorodon and Nassula among the Holotricha, Chilodon and Phascolodon 

 among the Hypotricha, and apparently in the solitary case of Polykrikos in 

 the group of the Peritricha. 



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