The nucleus is single: in Hemiophrya it is at first horseshoe-shaped, but is subsequently much branched, and resolved into thicker portions connected by slender filaments; and in Dendrocometes it branches and extends unbroken through the colony and creeping stolon. It has a nuclear membrane with dense contents: the chromatin is disposed in fibrils when the nucleus divides. Paranuclei have been observed in a few, e.g. Podophrya limbata, Acineta fetida. There is always one contractile vacuole and sometimes more. A special vacuolar duct is found in Podophrya Steinii, P. Wrzesniowskii and Dendrocometes. The contraction of the vacuoles appears to be slow in some instances. Want of oxygen causes them to become much distended. The presence of a large bubble of gas (Carbon dioxide ?) and its gradual absorption has been recorded in a Sphaerophrya (Engelmann). Trichocysts are said to occur in two species of the genus Ophryodendron.
Reproduction takes place by fission or gemmation, external or internal. Fission is universal in the genus Sphaerophrya; it has been observed in Urmda, in Podophrya libera, P. fixa, P. mollis and Acineta mystacina2. One of the parts is set free and retracts its tentacles completely except in Sphaerophrya, and becomes ciliate. The fission-product in Urmda and Acineta mystacina is decidedly smaller than its parent. External gemmation is characteristic of the genera Hemiophrya and Ophryodendron. In the former the buds are produced on the anterior face of the body, and so far as is known are ciliated on one aspect only. They have been observed with tentacles developed before detachment. As to the latter, the bud is non-ciliate, elongate, with a very mobile tubular process or neck; it is often termed 'lageniform zooid.' Internal gemmation is very general, and it occurs in Ophryodendron side by side with external. The process begins in Podophrya quadripartita by the formation of a depression which widens at its inner end, but remains permanently open. It is open also in Trichophrya, but in Dendrocometes its aperture closes. The closed brood cavities of other Acinetaria may be formed in a similar manner, but the fact has not been observed.
The bud is produced as a process of the floor of the depression or cavity. When it is single, as is commonly the case, it is of large size relative to its parent; when there are several buds, e. g. four in Acineta fetida, they are smaller, and a difference of size has been noted in various instances, dependent probably on the number formed1. The nucleus of the bud, whether external or internal, has been seen to originate by fission of the parental nucleus. It is beyond doubt always so formed. The bud or embryo, as it is sometimes called, is ciliated, in some instances uniformly, sometimes on one aspect only, e. g. Hemiophrya, sometimes in a zone, e. g. Podo-phrya quadripartite Dendrocometes, Dendrosoma, and then the cilia appear to be disposed not infrequently in a spiral. The passage of the embryo into the parent form has been traced in several instances. That of Dendrosoma, which is sessile, develops suctorial tentacles over the whole free surface before a vertical stem begins to grow out2.
1 Maupas, op. cit. p. 328, compares the arms of Dendeocometes to a bundle of united tentacles. On p. 356 he states that Koch has found that 'the proboscis of Ophryodendron has an identical structure.'Koch's work, 'Zwei Acineten auf Plumularia setacea (Ellis),' Jena, 1876, has not been accessible to me.
2Saville Kent describes the process in P. mollis as taking place thus: the animal fixes itself by means of two tentacles in addition to its own peduncle; the two tentacles fuse and become the peduncle of the new animal which is formed by the division of the doubly attached body. He remarks that unless witnessed from the beginning the process would be interpreted as one of conjugation. See 'Manual of Infusoria,' p. 821. Gruber states (Z. W. Z. xxxvi. p. 118) that he has seen gemmation, fission, and the detachment of small fission-products in Acineta mystacina.
Conjugation has been observed in some instances; it may be temporary, as in Dendrocometes, or permanent3. In Podophrya pyrum a formation of internal buds has been recorded as taking place immediately after it. Encystation under unfavourable circumstances is frequent, the Acine-tarian retracting its tentacles; binary fission of the contents of the cyst has been seen.
The Acinetaria are microscopic in size. The colony of Dendrosoma may attain a height of 1/10 in.; and the zooids of Ophryodendron abie-tinum a length of 1/30 in.
1 See Claparede and Lachmann, Etudes sur les Infusoires, etc. ii. 1860-1, in Podophrya Pyrum, p. 123; P. quadripartita, p. 120-1; in Acineta fetida, Maupas, A. Z. Expt. ix. p. 320-1. Embryoes contained within pocket-like diverticula of the cuticle at the aperture of the lorica have been observed in Acineta tuberosa by Hertwig, M. J. i. p. 68, note 1, and in A. mystacina, cf. Saville Kent, Manual of the Infusoria, ii. p. 834. Hertwig suggests (loc. cit.) that they are due to a precocious fission of an endogenously-formed embryo, and states that the nucleus of the bud in Hemi-ophrya gemtnipara may commence to branch, the first indication of budding, before the bud itself is detached. According to Saville Kent (op. cit. p. 844) external diverticula containing each an embryo are found singly or in groups near the apices of the stems in Dendrosoma. He never saw these buds become ciliate, but observed them with suctorial tentacles. Fraipont has given an account of the formation of external capsules, ' diverticules generateurs,' in Acineta divisa. They are pyriform, curved, with a terminal orifice: the protoplasm within them appears to be a process of the periphery of the body. It gives origin to an ovoid bud with a zone (?) of cilia.
The process of protoplasm may give origin to a second, or to a series of such buds. See Bull. Acad. Roy. Belg. 44. 1877, P- 805.
3 Whether spores are ever formed or not is an open question. Fraipont observed an encysted Podophrya ( = Hemiophryd) truncata resolved into minute spheres (op. cit. ante, 45, 1878, p. 296), and Claparede and Lachmann state that Urnula Epistylidis gives rise to them (op. cit. ante, p. 210), but they suggest also that the spores belong to a Chytridium (op. cit. p. 255, note).
3 In Dendrocometes a bridge of protoplasm is formed: currents pass from one individual to the other, and the nuclei are subsequently broken up and regenerated (Plate). Conjugation might be mistaken for fission; see note 2, p. 827 ante.