Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/866

Rh 842 P H T Z A [LOBOSA. podia are lobose, ranging in form from mere wave-like bulgings of the surface to blunt finger-like processes, but never having the character of filaments either simple, arborescent, or reticulate. Fusions of two individuals (conjugation) have been observed in a fi o, ,1 ? nous Lol)08a - 1, 2, 3. Dactylos)&amp;gt;h:era (Amoeba) poli/podla, M. .Schultze, in three successive stages of division; the changes indicated occupied fifteen minutes, a, nucleus ; b, contractile vacuole (copied from ultze, in Arclurf. HI, kiosk. Anal.). 4. Amoeba princejtx, Ehr. ided granules are food-particles). 5. Peloinyxa paluxtrw, Orceff after Greeff), an example with comparatively few food-particles (natural size ^tn inch in length}. C. Portion of a Pelornyxa more hichl y niasrni- led. a, clear superficial zone of protoplasm (so-called &quot; exoplasm &quot;) b vacuoles extremely numerous; c, lobose pseudopodium ; d, a similar pseudopodium ; e, nuclei ; /, &quot; refractive bodies &quot; (reproductive ?) ; scattered it in the protoplasm are seen numerous cylindrical crystals 7 Arcella vulgarit, Ehr. a, shell ; b, protoplasm within the shell ; c, extended protoplasm in the form of lobose pseudopodia; &amp;lt;l, nuclei; e, contractile vacuole; the dark bodies unlettered are gas vacuoles. 8. Cochlio- xtium pi-lluciduin, Hert. and Less, a, nucleus surrounded by a hyaline sometimes mistaken for the nucleus, whilst the latter is termed nucleolus. few cases, but not fusions of many individuals so as to form plasmodia ; nevertheless the size attained by the naked protoplasm by pure growth is in some cases considerable, forming masses readily s by the naked eye (Peloinyxa). The presence of more than one nucleus is a frequent character. A contractile vacuole may or may not be present. The formation of sporocysts and of chlamydo- sporcs (coated spores) has not been observed in any species, but naked spores (nagellulse or amoebulee) have been with more or less certainty observed as the product of the breaking up of some species (Amoeba ? Pelomyxa). The cyst phase is not unusual, but the cyst appears usually to be a hypnocyst and not a sporoeyst. In the best observed case of spore-production (Pelomyxa) the spores were apparently produced without the formation of a cyst. Repro duction is undoubtedly most freely effected by simple li.ssion (Amu ba) and by a modified kind of bud-fission (Arcella). Fresh water and marine. Two orders of the Lobosa are distinguished in accordance with the presence or absence of a shell. ORDER 1. NUDA. Characters. Lobosa devoid of a shell. Genei a. Amceba, Auct. (Fig. IV. 4); Uuramcela, Leidy (with a villous tuft at one end, Wallich s A. rillosa) ; (Jori/cia, Duj. (low, ridge-like pseudopodia) ; Lithamceba, Laukcster (Fig. V.); Dina- mceba, Leidy (92) (covered with short still processes) ; Hyalodiscus, H. and L. ; Plakopus, F. E. Schultze; Dudylosplixra, H. and L. (Fig. IV. 1, 2, 3); Pelomyxa, Greelf (Fig. IV. 5, 6) ; Ainphizondht, Greeft (forms a gelatinous case which is broken through by the pseudopodia). ORDER 2. TESTACEA. Characters. Lobosa which secrete a shell provided with an aperture from which the naked protoplasm can be protruded. The shell is either soft and membranous, or strengthened by the in clusion of sand-particles, or is hard and firm. Genera. Cochliopodium (Fig. IV. 8), 11. and L. ; Pi/xidictila, Ehr. ; Arcclla, Ehr. (Fig. IV. 7); Hyalosphenia, Stein; Quad- rultt, F. E. Schultze (shell membraneous, areolated) ; Difflugia, Leclerc (shell with adventitious particles). Further remarks on the Lobosa.- The Lobosa do not form a very numerous nor a very natural assemblage. Undoubtedly some oF the forms which have been described as species of Amn-ba are amoeba forms of Mycetozoa ; this appears to be most probably the case in parasitic and stercoricolous forms. But when these are removed, as also those Proteomyxa which have pseudopodia of varying character, at one time lobose and at another filamentous, we have left a certain small number of independent lobose Gymnomyxa which it is most convenient to associate in a separate group. We know very little of the production of spores (whether it actually obtains or not) or of developmental phases among these Lobosa. The common Amcebre are referable to the species A. princcps, A. lobosa, Dactylosphsera polypodia, Ouramosba fillosa. Of none of these do we know certainly any reproductive phenomena excepting that of fission (sec Fig. IV. 1, 2, 3). Various statements have been made pointing to a peculiar change in the nucleus and a production of spores having the form oi minute Amoeba:, arising from that bod} ; but they cannot be considered as established. Whilst the observed cases of supposed reproduc tive phenomena are very few, it must be remembered that we have always to guard (as the history of the Ciliata has shown, sec below) against the liability to mistake parasitic amojbuhe and flagellulaj for the young forms of organisms in which they arc merely parasitic. The remarkable Pelomyxa jmlustris of Greelf (32) was seen by him to set free (without forming a cyst) a number of aiiKebuke which he considers as probably its young. ]lr Weldon of St John s College, Cambridge, has observed the same pheno menon in specimens of Pelomyxa which made their appearance in abundance in an aquarium in the Morphological Laboratory, Cambridge. It seems probable that the amoebula 1 in this case are not parasites but spore-like young, and this is the best observed case of such reproduction as yet recorded in the group. Arcella is remarkable for the production of bud-spores, which may be considered as a process intermediate between simple fission and the complete breaking up of the parent body into spores. As many as nine globular processes are simultaneously pinched oil from the protoplasm extruded from the shell of the Arcella ; the nuclei (present in the parent Arcella to the number of two or three) have not been traced in connexion with this process. The buds then be come nipped off, and acquire a shell ami a contractile vacuole (33). The presence of more than one nucleus is not unusual in Lobosa, and is not due to a fusion of two or more uninuclear individuals, but to a multiplication of the original nucleus. This has been observed in some Amcebaj (A. princcj&amp;gt;s ) as well as Arcella. Pelomyxa (Fig. IV. 6) has a great number of nuclei like the llelio- zoon, Actinosphaerium (Fig. VIII.). Pelomyxa is the most highly differentiated of the Lobosa. The highly vacnohited character of its protoplasm is exhibited in a less degree by Lithanieeba and resembles that of Heliozoa and Radiolaria. Besides the numerous nuclei there are scattered in the protoplasm strongly refringent bodies (Fig. IV. 6, /), the significance of which has not been ascertained. The superficial protoplasm is free from vacuoles, hyaline, and extremely mobile. Occasionally it is drawn
 * r Auerbach). a, nucleus ; b, c, vacuoles (one or more contractile the