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DISTINCTION BETWEEN PROTOZOA AND PROTOPHYTES. 47bearing members of these latter, including in the plant series the independentunicellular Protophytes, and the so-called zoospores or antherozooids ofAlgae and Confervae, and in the animal one chiefly the Pantostomatous andsome Eustomatous Flagellata, that a difficulty in determination is likely toarise. To one accustomed, however, by a long practical acquaintance with thecharacteristic motions of these minute beings, there is at once recognizable aso to say, method in the manner of the locomotion associated withcertain,the animal organism which is under no circumstances encountered in thecase of the plant. In the latter instance, citing as examples the motilephases of Volvox, Chlamydomonas, or Protococcus, it will be found that theyswim, as it were, blindfold through the water, stumbling and strikingagainst their fellow forms or any other object that may be opposedto their aimless course. In the animal types, on the contrary, as illustratedby a Euglena, Monas, or Heteromita, there is no such absence ofpurpose in their movements ; tentative, well-controlled progress in variousdirections, and intelligent deviations, or, as it were, tackings backwards orto either side being continually displayed.Objects lying in their path are,again, carefully passed over or avoided, similar conduct being likewiseobservable in their encounter with comrades of the same or diverse species.Under these latter conditions there is often, moreover, exhibited a distinctappreciation of the society of their associates, this phenomenon being morealluded to in connection with the two forms described hereafterespeciallyunder the respective names of Heteromita ludibunda and Chloraster marina.One other accessory character, scarcely yet, perhaps, sufficiently investigatedfor recording as an undeviating diagnostic feature of distinction, isconnected with the presence or absence of the rhythmically expandingand contracting space, sometimes single and sometimes multiple, situatedat various points such position in a given specific form being invariablydefinite within the cortical substance of the organism, and designatedthe contractile vesicle or vacuole. Among the representatives of the animalseries, this structure, excepting in certain Opalinidae, would appear to beconstantly present, while in vegetable forms it would seem to be as invariablyabsent. From the explanation of the character and functions of this specialorgan or structure given in the succeeding chapter,it is reasonable topredicate that it is an accompaniment only of animal organization. Asthere shown, this pulsating vessel is continually replenished from the fluidelement imbibed by the organism with the solid food-particles, or throughthe ciliary or otherwise produced currents, the same fluid often travellingtowards and debouching into the vessel in question by well-defined canallikechannels. According to some observers, including Stein, contractilevesicles are common also to several undoubted plant forms, such as Volvoxand Protococcus. The closest investigation in this direction on the author'spart, and as accomplished with the aid of a magnifying power of I- or 2000diameters which renders these structures distinctly visible in organisms offar more minute size has, however, entirely failed to substantiate the