264 ORDER FLAGELLATA-PANTOSTOMATA. 



granular band or groove near the anterior extremity, erroneously supposed by Stein 

 and Cienkowski to represent a distinct oral aperture. In its free-swimming con- 

 dition, again, the present type usually exhibits but little alteration of its sedentary 

 shape, while S. guttiila is subject under like conditions to the most protean 

 metamorphoses. With respect to the reproductive process, both Stein and 

 Cienkowski accord in attributing to S. guttula the production of endogenous spore- 

 masses formed from an isolated central portion of the animalcule, while in Physo- 

 monas socialis, as here shown, the entire body-mass becomes split up to produce 

 the sporular elements. 



Detailing at length the more important features of the reproductive phenomena 

 of the present type, as observed by the author, it may be remarked that, in addition 

 to the ordinary process of longitudinal fission, the encystment of animalcules, accom- 

 panied or not by the coalescence of two zooids, and succeeded by the breaking 

 up of the encysted body into comparatively large macrospores, has been witnessed, 

 as also the escape of these latter as simple uniflagellate monadiform germs closely 

 corresponding in contour with the type upon which Perty has instituted the genus 

 Plenromonas, already described. Various phases of this reproductive process will 

 be found delineated at PI. XIV. Figs. 43-45. As there shown, the animalcules 

 mostly remain attached to their pedicles throughout these transformations, the last- 

 named structure indeed losing its vitality and becoming as rigid and indurated as 

 the peripheral wall of the encystment. The example indicated by the letter c in the 

 group delineated at Fig. 43, is of especial interest, it indicating by the two pedicles 

 that support the single cyst, that this latter structure has been formed by the coales- 

 cence of two zooids in their attached condition : the two others (a and b) close to 

 this are as evidently the result of solitary encystment. As made apparent by the 

 principal group illustrative of this species (Fig. 37), it may be described as eminently 

 social, numbers being usually found crowded together on the vegetable debris 

 obtained from those localities which it favours, and these exhibiting by their com- 

 bined restless movements as they sway to and fro on their slender elastic pedicles 

 their vibrating flagella producing strong currents in the surrounding water a 

 scene of animation that is scarcely rivalled by the social groups of the larger and 

 more highly organized representatives of the Ciliata. Although the adult zooids 

 of the same colony correspond in size, a considerable difference is found to subsist 

 between those derived from separate localities. The two extremes in this respect 

 are recorded in the foregoing diagnosis ; an intermediate calibre, that of 1-3000", 

 would, however, appear to be most common. 



The binary character of the contractile vesicle in this species was determined 

 on numerous occasions, but is necessarily difficult to recognize unless the animal- 

 cules face the observer in such a way that the two vesicles are placed side by 

 side, and not one in front of the other. On first examining this type, and 

 mastering the details of its organization and reproductive phenomena, in the 

 year 1871, the writer was disposed to identify it with the Bodo socialis of Ehren- 

 berg. The present specific title is even now retained with reference to the proba- 

 bility of such identity. A species of Physomonas, closely agreeing with the present 

 form, but having the body more globular and with that anteriorly emarginate 

 aspect caused by the greater extension of one side of the anterior border less marked, 

 has been obtained by the author in sea-water, at St. Heliers, Jersey : this type or 

 variety may be provisionally distinguished by the title of Physomonas marina. The 

 characters afforded by the flagella, together with the dimensions of the body, coincide 

 precisely with those of P. socialis. 



GENUS II. CLADONEMA, S. K. 

 (Greek, klados, branch ; nema, thread.) 



Animalcules forming social colony-stocks, irregularly pyriform, with an 

 obliquely truncate anterior border, attached singly to the extremities of a 

 perfectly flexible, hyaline, slender and thread-like, branching pedicle ; 



