Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/889

ACINETARIA.] PROTOZOA 865 be learnt from an examination of Figs. XXIII. , XXIV., XXV., and the explanations appended to them. CLASS VI. ACINETARIA, Lankester ( Tentaculifera, Huxley). Characters. Highly specialized Corticate Protozoa, probably derived from Ciliata, since their young forms are provided with a more or less complete investment of cilia. They are distinguished by having no vibratile processes on the surface of the body in the adult condition, whilst they have few or many delicate but firm FIG. XXVI. Acinetaria. 1. Rhyncheta cyclopum, Zenker. , nucleus ; b, contractile vacuole ; only a single tentacle, and that suctorial ; x 150. Parasitic on Cyclops. 2. Sphierophrya, urostylx, Maupas ; normal adult ; x 200. a, nucleus ; b, contractile vacuole. Parasitic in TJrostyla. 3. The same dividing by transverse fission, the anterior moiety with tem porarily developed cilia, a, nucleus ; 6, contractile vacuole. 4, 5, 6. Sphxrophrya stentorea, Maupas ; x 200. Parasitic in Stentpr, and at one time mistaken for its young. 7. Trichophrya epistylidis, Cl. and L. ; x 150. a, nucleus ; b, contractile vacuole. 8. Hemiophrya gemrni- para, Hertwig ; x 400. Example with six buds, into each of which a branch of the nucleus a is extended. 9. The same species, showing the two kinds of tentacles (the suctorial and the pointed), and the con tractile vacuoles 6. 10. Ciliated embryo of Podophrya Steinii, Cl. and L. ; x 300. 11. Acineta grandis, Saville Kent ; x 100 ; showing pedun- culated lorica, and animal with two bunches of entirely suctorial tentacles. a, nucleus. 12. Svhxrophrya mayna, Maupas ; x 300. It has seized with its tentacles, and is in the act of sucking out the juices of six examples of the ciliate Colpoda parvifrons. 13. Podophrya elonyata, Cl. and L.; x 150. a, nucleus ; b, contractile vacuole. 14. Hemiophrya Benedemi, Fraip.; x 200; the suctorial tentacles retracted. 15. Dendrocometeg rradoxus, Stein ; x 350. Parasitic on Gammarus puU x. a, nucleus ; , contractile vacuole ; c, captured prey. 16. A single tentacle of Podophrya; x 800. (Saville Kent.) 17-20. Dendrosoma radians, Ehr.: 17, free-swimming ciliated embryo, x 600 ; 18, earliest fixed condition of the embryo, x 600 ; 19, later stage, a single tentaculiferous process now developed, x 600 ; 20, adult colony ; c, enclosed ciliated embryos ; d, branching stolon ; e, more minute reproductive (?) bodies. 21 ""Jw dendron pedicellatum, Hincks ; x 300. tentacle-like processes, which are either simply adhesive or t - .iar and suctorial. In the latter case they are provided at tli r ex tremity with a sucker-disk and have contractile walls, whereas in the former case they have more or less pointed extremities. The Acinetaria are sedentary in habit, even if not, as is usual, per manently fixed by a stalk. The nucleus is frequently arboriform. Reproduction is effected by simple binary fission, and by a modified fission (bud-fission) by which (as in Reticularia and Arcella) a number of small bud-like warts containing a portion of the branched parental nucleus are nipped off from the parent, often simul taneously (Fig. XXVI. 8). These do not become altogether dis tinct, but are for a time enclosed by the parental cell each in a sort of vacuole or brood-chamber, where the young Acinetarian develops a coat or band of cilia and then escapes from the body of its parent (Fig. XXVI. 10, 17). After a brief locomotive existence, it becomes sedentary, develops its tentacles, and loses its cilia. The Acinetaria have one or more contractile vacuoles. Their nutrition is holozoic. The surface of the body in some cases is covered only by a delicate cuticle, but in other cases a definite membranous shell or cup (often stalked) is produced. Freshwater and marine. See Fraipont (89). ORDER 1. SUCTORIA, Kent. A greater or less proportion or often all of the tentacles are suctorial and terminated with sucker-like expansions. Genera. Rhyncheta, Zenker (stalkless, naked, with only one tentacle ; epizoic on Cyclops ; Fig. XXVI. 1) ; Urnula, C. and L. ; Sphserophrya, C. and L. (naked, spherical, with distinctly capitatq tentacles only ; never with a pedicle ; parasitic within Ciliata, supposed young ; Fig. XXVI. 2-6, 12); Trichophrya, C. and L. (as Sphazrophrya, but oblong and temporarily fixed without a pedicle); Podophrya, Ehr. (naked, solitary, globose, ovate or elongate, fixed by a pedicle ; tentacles all suctorial, united in fascicles or distri buted irregularly; Fig. XXVI. 10, 13, 16) ; Hemiophrya, S. Kent (as Podophrya, but the tentacles are of the two kinds indicated in the definition of the group ; Fig. XXVI. 8, 9, 14); Podocyathus, S. Kent (secreting and inhabiting stalked membranous cups or loricse ; ten tacles of the two kinds) ; Solcnophrya, C. and L. (with a sessile lorica ; tentacles only suctorial) ; Acineta, Ehr. (as Sofenophrya, but the lorica is supported on a pedicle; Fig. XXVI. 11); Dendro- cometes, Stein (cuticle indurated ; solitary, sessile, discoid ; tentacles peculiar, viz., not contractile, more or less branched, root-like, and perforated at the extremities and suctorial in function ; Fig. XXVI. 15). Dendrosoma, Ehr. (forming colonies of intimately fused individuals, with a basal adherent protoplasmic stolon and upstanding branches the termination of which bear numerous capi tate suctorial tentacles only ; Fig. XXVI. 17-20). OiiDER2. XOX-SUCTORIA, Lankester ( = Adinaria, Kent). Characters. Tentacles filiform, prehensile, not provided with a sucker. Genera. Ephclota, Str. "Wright (solitary, naked, pedunculate, with many flexible inversible tentacles) ; Adinocyathus, S. Kent ; Ophryodcndron, C. and L. (sessile, with a long, extensile, anterior proboscis bearing numerous flexible tentacles at its distal extremity ; Fig. XXVI. 21) ; Adnetopsis, Robin (ovate, solitary, secreting a stalked lorica ; from the anterior extremity of the animal is deve loped a proboscis-like organ which does not bear tentacles). Further remarks on the Acinetaria. The independence of the Acinetaria was threatened some years ago by the erroneous view of Stein (79) that they were phases in the life-history of Vorticellidse. Small parasitic forms (Sphserophrya) were also until recently regarded erroneously as the "acinetiform young" of Ciliata. They now must be regarded as an extreme modification of the Protozoon series, in which the differentiation of organs in a unicellular animal reaches its highest point. The sucker-tentacles of the Suctoria are very elaborately constructed organs (see Fig. XXVI. 16). They are efficient means of seizing and extracting the juices of another Protozoon which serves as food to the Acinetarian. The structure of Dendrosoma is remarkable on account of its multicellular character and the elaborate differentiation of the reproductive bodies. The ciliation of the embryos or young forms developed from the buds of Acinetaria is an indication of their ancestral connexion with the Ciliata. The cilia are differently disposed on the young of the various genera (see Fig. XXVI. 10, 17).

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