This group, despite a superficial resemblance to the Heliozoa, show a close affinity to the Ciliata; the nuclear apparatus is usually double though a micronucleus is not always seen; the young are always ciliated, and the mode of conjugation is identical in all cases hitherto studied. Most of the genera are attached by a chitinous stalk (Fig. 61), continued in Acineta into a cup or "theca" surrounding the cell. The pellicle is firm, often minutely shagreened or "milled" in optical section by fine radial processes, whether superficial rods or the expression of the meeting edges of radial alveoli is as yet uncertain. The pellicle closely invests the ectosarc, is continued down into a tubular sheath, from the base of which the tentacle rises, and upwards to invest the tentacle, and is even prolonged into its cavity in Choanophrya, the only genus where the tentacles are large enough for satisfactory demonstration. These organs may be one or more, and vary greatly in character. They may be (1) pointed for prehension, puncture, and suction (Ephelota, Fig. 61, 5); (2) nearly cylindrical, with a slightly "flared" truncate apex (Podophrya, Fig. 61, 1a); (3) filiform with a terminal knob; (4) "capitate" (Acineta, Fig. 61, 2); (5) bluntly truncate and capable of opening into a wide funnel for the suction of food[[179]] (Choanophrya, Fig. 62; Rhyncheta, Fig. 61, 3). Their movements, too, are varied, including retraction and protrusion, and a degree of flexion which reaches a maximum in Rhyncheta (Fig. 61, 3), whose tentacle is as freely motile as an elephant's trunk might be supposed to be were it as slender in proportion to its length. They are continued into the body, and in Choanophrya may extend right across it. In Podophrya trold the pellicle rises into a conical tube about the base of the tentacle, which is retracted through it completely with the prey in deglutition. In Dendrocometes, Dendrosoma, and Ophryodendron (Fig. 61, 4, 9, 7), the tentacles arise from outgrowths of the cell-body.
Fig. 61.—Various forms of Suctoria. 1, a and b, two species of Podophrya; c, a tentacle much enlarged; 2, a, Acineta jolyi; 2, b, A. tuberosa, with four ciliated buds; in 6 the animal has captured several small Ciliata; 8, a, a specimen multiplying by budding; 8, b, a free ciliated bud; 9, a, the entire colony; 9, b, a portion of the stem; 9, c, a liberated bud. a, Organism captured as food; b.c, brood-cavity; bd, bud; c.vac, contractile vacuole; l, test; mg.nu, meganucleus; mi.nu, micronucleus; nu, nucleus; t, tentacle. (From Parker and Haswell, after Bütschli and Saville Kent.)
The mechanism of suction is doubtful; but from the way particles from a little distance flow into the open funnels of Choanophrya, it may be the result of an increase of osmotic pressure. The external pellicle of the tentacles is marked by a spiral constriction,[[180]] which may be prolonged over the part included in the sheath. The endosarc is rich in oil-drops, often coloured, and in proteid granules which sometimes absorb stains so readily as to have been named "tinctin bodies." It usually contains at least one contractile vacuole.
In Dendrocometes (and perhaps others) the whole cell may become ciliated, detach itself and swim off; this it does when its host (Gammarus) moults its cuticle.
In fission or budding we have to distinguish many modes. (1) In the simplest, after the nuclear apparatus has divided, the cell divides transversely; the distal half acquires cilia and swims off to attach itself elsewhere, while the proximal remains attached. The tentacles have previously disappeared and have to be formed afresh in both. (2) More commonly fission passes into budding on the distal face; a sort of groove deepens around a central prominence which becomes the ciliated larva (Fig. 62, em); the tentacles of the "parent" are retained. This process passes into (3) "internal budding," where a minute pit leads into a bottle-shaped cavity.[[181]] (4) Again, the budding may be multiple, the meganucleus protruding a branch for each bud, while the micronucleus, by successive divisions, affords the supply requisite. Sphaerophrya (Fig. 61, 6) and Endosphaera multiply freely by fission within their Ciliate hosts, and were indeed described by Stein as stages in their life-cycle. Conjugation of the same type as in most Ciliates has been fully worked out in Dendrocometes alone, by Hickson,[[182]] who has found the meganuclei (though destined to disorganisation) conjugate for a short time by the bridge of communication before the reciprocal conjugation of the micronuclei.
We have referred to the endoparasitism of two genera. Amoebophrya lives in several Acanthometrids, and in the aberrant Radiolarian Sticholonche (see p. [86]). The attached species are some of them indifferent to their base; others are only found on Algae, or again only epizoic on special Metazoan hosts, or even on special parts of these. Thus Rhyncheta is only found on the couplers of the thoracic limbs of Cyclops, and Choanophrya on the ventral surface of its head and the adjoining appendages.
Fig. 62.—Choanophrya infundibulifera. A, adult; B-D, tentacles in action in various stages; E, tentacle at rest; F, young, just settled down, a, a, a, Tentacles in various stages of activity; c, central cavity; c.v, contractile vacuole; em, ciliated embryo showing contractile vacuole and nucleus; f, spiral ridge; m, muscular wall of funnel; n, nucleus; tr, opening of funnel. (A-D, F, modified after Zenker; E, original.)
We owe our knowledge of this group to the classical works of Ehrenberg, Claparède and Lachmann, Stein, R. Hertwig, and Bütschli. Plate has shed much light on Dendrocometes, and Hickson has studied its conjugation. Ischikawa[[183]] has utilised modern histological methods for the cytological study of Ephelota bütschliana. René Sand has written a useful, but unequal, and not always trustworthy monograph of the Order,[[184]] containing an elaborate bibliography.