DISTINCTION BETWEEN PROTOZOA AND PROTOPHYTES. 47 



bearing members of these latter, including in the plant scries the indepen- 

 dent unicellular Protophytes, and the so-called zoospores or antherozooids of 

 Algae and Conferva, and in the animal one chiefly the Pantostomatous and 

 some Eustomatous Flagellata, that a difficulty in determination is likely to 

 arise. To one accustomed, however, by a long practical acquaintance with the 

 characteristic motions of these minute beings, there is at once recognizable a 

 certain, so to say, method in the manner of the locomotion associated with 

 the animal organism which is under no circumstances encountered in the 

 case of the plant. In the latter instance, citing as examples the motile 

 phases of Volvox, CJdamydomonas, or Protococcus, it will be found that they 

 swim, as it were, blindfold through the water, stumbling and striking 

 against their fellow forms or any other object that may be opposed 

 to their aimless course. In the animal types, on the contrary, as illus- 

 trated by a Eiiglena, Monas, or Heteromita, there is no such absence of 

 purpose in their movements ; tentative, well-controlled progress in various 

 directions, and intelligent deviations, or, as it were, tackings backwards or 

 to either side being continually displayed. Objects lying in their path are, 

 again, carefully passed over or avoided, similar conduct being likewise 

 observable in their encounter with comrades of the same or diverse species. 

 Under these latter conditions there is often, moreover, exhibited a distinct 

 appreciation of the society of their associates, this phenomenon being more 

 especially alluded to in connection with the two forms described hereafter 

 under the respective names of Heteromita ludibiuida and Chloraster marina. 

 One other accessory character, scarcely yet, perhaps, sufficiently investi- 

 gated for recording as an undeviating diagnostic feature of distinction, is 

 connected with the presence or absence of the rhythmically expanding 

 and contracting space, sometimes single and sometimes multiple, situated 

 at various points — such position in a given specific form being invariably 

 definite — within the cortical substance of the organism, and designated 

 the contractile vesicle or vacuole. Among the representatives of the animal 

 series, this structure, excepting in certain Opalinidae, would appear to be 

 constantly present, while in vegetable forms it would seem to be as invariably 

 absent. From the explanation of the character and functions of this special 

 organ or structure given in the succeeding chapter, it is reasonable to 

 predicate that it is an accompaniment only of animal organization. As 

 there shown, this pulsating vessel is continually replenished from the fluid 

 element imbibed by the organism with the solid food-particles, or through 

 the ciliary or otherwise produced currents, the same fluid often travelling 

 towards and debouching into the vessel in question by well-defined canal- 

 like channels. According to some observers, including Stein, contractile 

 vesicles are common also to several undoubted plant forms, such as Volvox 

 and Protococcus. The closest investigation in this direction on the author's 

 part, and as accomplished with the aid of a magnifying power of i- or 2000 

 diameters — which renders these structures distinctly visible in organisms of 

 far more minute size — has, however, entirely failed to substantiate the 



