41 8 ORDER FLAGELLATA-EUSTOMATA. 



The difference between this and the preceding species appears to be only one of 

 size, and ahnost too shght for separate distinction. 



Genus III. DISTIGMA, Ehrenberg. 



Animalcules free-swimming, highly elastic and changeable in form ; 

 flagella terminal, two in number, one long and one short, both vibratile ; oral 

 aperture close to the base of the flagella, succeeded by a long, tubular 

 pharyngeal passage ; endoplasm transparent, usually enclosing two ante- 

 riorly developed, minute, eye-like pigment-spots ; contractile vesicle and 

 endoplast conspicuous. 



Distigma proteus, Ehr. Pl. XXI. Figs. 46-51. 



Body highly metabolic, scarcely ever presenting the same contour, usually 

 more or less elongate, with irregular constrictions and distensions ; longer 

 flagellum nearly equalling the body in length, the shorter one scarcely 

 one-quarter that length ; endoplasm transparent, enclosing numerous dark- 

 coloured refringent corpu.scles whose positions are constantly shifting from 

 one extremity to the other in accordance with the peristaltic motions of the 

 body ; two minute, blackish, eye-like pigment-spots usually developed at 

 the anterior extremity ; tubular pharynx slender, greatly prolonged ;^con- 

 tractile vesicle conspicuous, located close to the termination of the pharynx ; 

 endoplast ovate, subcentral. Length 1-580" to 1-240". 



Hab. — Pond water, among Leniuce. 



Stein * refers this species to the genus Astasia, here and elsewhere more generally 

 retained for the reception only of certain monoflagellate types. He further pro- 

 poses, in his index to the figures given, to identify the countless protean forms 

 assumed by this animalcule with the Proteus tenax of O. F. Miiller and Distigma 

 proteus et tenax, Astasia flavicans et pusilla, and Monas pundum of Ehrenberg ; 

 these last three hypothetic species being more especially identified with the earlier 

 stages of its growth. In neither of the two first-named forms did Ehrenberg detect 

 the presence of any flagellate appendage, and it was assumed by him that such 

 do not exist ; the movements of the animalcule were further described by this 

 authority as simply repent and peristaltic. As shown by Stein, however, it is only 

 the older zooids that lose their flagella and lead a repent life, the younger and more 

 normal ones possessing two conspicuous but unequal sized flagella, as indicated 

 in the preceding diagnosis. This elder creeping phase corresponds evidently with 

 the amoeboid one assumed by the majority of the Flagellata as a preliminary 

 step to the act of coalescence or encystment. The peculiar peristaltic movements 

 exhibited by the representatives of this species, both during their natatory and repent 

 states, coincide closely with those that characterize the Gregarinada, and are, with 

 the exception of Eutreptia, met with nowhere else among the Infusoria. The 

 Proteus tenax of O. F. Miiller, while greatly resembling the repent phase of the present 

 species in outward form, is possibly, from the description given of the parenchyma — 

 " pellucid and filled with black granules " — a species of Gregarina, identical probably 

 with the one infesting various species of Cyclops, and not unfrequently found free 

 in the water containing this abundantly distributed Entomostracon. As such an 

 independent Gregarine type Stein has indeed previously recognized it, describing 

 itf under the title of Monocystis tenax. The Distigma viridis of Ehrenberg would 



* • Infusionsthiere,' Abth. iii., 1878. f Ibid., ii., 1867, pp. 7 and 8. 



