ORDER FLAGELLATA-EUSTOMATA. 429 



Fam. VIII. ANTSONEMID.E3, S. K. 



Animalcules ovate or elongate, free-swimming or temporarily adherent ; 

 flagella two in number, the anterior one or "tractellum" locomotive and 

 vibratile, the posterior one or " gubernaculum " used for steering or trailed 

 inactively in the rear during natation, adherent or anchorate by its pos- 

 terior extremity in the sedentary condition ; oral aperture distinct, mostly 

 associated with a well-defined tubular pharynx ; endoplasm transparent, 

 granular. Inhabiting salt and fresh water. 



The representatives of this well-defined family group exhibit a type of structure 

 and habits of life strictly analogous to those already recounted of the Heteromitidag, 

 the one highly important differential feature being that a well-defined and often 

 complex oral apparatus takes the place of the indefinite and dispersed inceptive area 

 characteristic of that family. This oral apparatus in certain members of the Aniso- 

 nemidae is so complex as to approach the formula typical of Nassula and various 

 other Holotrichous and Hypotrichous Ciliata, and there can be little doubt that 

 these animalcules must be regarded as occupying the foremost position among the 

 Stomatode Flagellata. A further development of the Anisonematous structural type 

 towards the Ciliate division of the Infusoria is afforded by Professor Clark's genus 

 Heteromastix, hereafter described, and in which an adoral fringe of cilia is added to 

 the two flagella. 



The peculiar modification of the Flagellate appendages of the Anisonemidae, shared 

 likewise by the Pantostomatous Heteromitidae, see p. 290, and the Cilio-flagellate 

 type Heteromastix just quoted, invites more extended attention. The desirability of 

 introducing a suitable term for the distinction of the posterior or trailing flagellum 

 used for the purpose of temporarily anchoring the animalcule to submerged 

 objects, or rudder- wise for the guidance of its course during natation in these 

 various types, was first recognized by Professor H. James-Clark,* who proposed to 

 distinguish such appendage by the appropriate title of a " gubernaculum." Irrespec- 

 tive of the distinction just alluded to, it has been more recently pointed out by 

 Professor E. Ray Lankester \ that even among uniflagellate organisms the single 

 flagellum may exercise two totally distinct functions, and merits in each case a 

 separate appellation. Thus, among all ordinary Flagellata such as Monas, Euglena, 

 and Astasia, the vibratory motion of the single flagellum draws the animalcule 

 after it through the water. The appendage possessing such a tractive function 

 Professor Lankester has proposed to distinguish by the title of a " tractellum." In 

 other flagellate organisms, as illustrated most familiarly by ordinary spermatozoa 

 and Bacteria, the filamentous appendage or appendages fulfils an entirely opposite 

 mission, its motions propelling the body in front of it through the liquid medium. 

 Where it thus acts as an organ of propulsion the authority last quoted confers upon 

 it the name of a " pulsellum." Among the free-swimming monoflagellate Infusoria 

 as at present known, where the locomotive appendage without exception fulfils 

 during natation the role of a tractellum, its recognition by such title in contra- 

 distinction to a propelling organ or pulsellum is uncalled for. With the biflagellate 

 types, however, exhibiting a differentiation of their two appendages corresponding 

 with that presented by the group now under consideration, Professor Lankester's 

 term as applied to the anterior locomotive flagellum, and as distinctive of the trailing 

 flagellum or gubernaculum of Professor Clark, becomes highly useful, and is here 

 unhesitatingly adopted. 



An exceptional instance among the Dimastigous Flagellata, in which one 

 appendage acts as a pulsellum or organ of propulsion, is afforded by Oxyrrhis 

 marina, described at pp. 427 and 428. Artificially detached zooids of the 



* 'Memoirs of the Boston Society of Natural History,' 1878. 

 t Biological Lectures, University College, London, 1876. 



