The Hypotricha have the dorsal aspect either naked or beset with fine setae, of which one or more are posterior in position, long, and occasionally compound. Cilia are confined to the flattened ventral aspect: in the two families Oxytrichidae and Euplotidae they take the form of cirri, either claw-like at their apex (uncini) or straight (styli), which are grouped in a definite manner1. The same two families and the genus Peritromus possess a peristome in the shape of an arcuate groove on the left-hand anterior edge of the body. The cilia in connection with it are disposed in special lines or bands; one band is always present, an adoral fringe of membranellae; a second, known aspraeoral, is very generally found near the dorsal edge. Two others, the endoral and paroral, are not so common. A vibratile membrane may take the place of the praeoral band. In the Peritricha the body is non-ciliate except in the Urceolarine genus Trichodinopsis; the cilia are confined to one or more encircling bands, or to a spiral band as in Vorticellina, Vaginicolina, and Ophrydina, which consists of membranellae (Biitschli). The right hand end of the spiral enters the mouth, and it is situate on a terminal retractile disc, which is protected in the retracted condition by the closing over of a raised wall or peristome, also by a thickened operculum attached below the peristome on one side (Pyxicola, Pachytrochd), or a moveable operculum within the lorica (Thuricola)2. Noteworthy peculiarities are the following: - the perioral contractile collar of the Peritrichan, Torquatella typica; the girdle of springing setae in Halteria\ the tentacles, long, scattered, retractile, of Actinobolus radians, four, short, knobbed, perioral, of Mesodinium pulex, which, though Acinetarian in aspect, are not suctorial3.

1 See Maupas, A. Z. Expt. (2), i. p. 574. Opalina is said by Zeller to be naked, but the superficial bands, supposed by him to be muscular, are probably cuticular. The chemical nature of the plastic cuticle is unknown: it is more resistent than protoplasm, but decomposes sooner or later in water.

2 For cilia as organs of adhesion, see Maupas, A. Z. Expt. (2), i. pp. 629-30. Note that membranellae are said to appear in Stentor as a continuous membrane subsequently broken up. A temporary fusion of cilia into vibratile bundles has been sometimes observed. See Geza Entz, Mitth. Zool. Stat. Naples, v. p. 332.

1The groups in question are: (I) frontal, anterior and on the animal's right; (2) ventral, sometimes replaced by rows of cilia; (3) anal s. transverse; (4) marginal, a fringing row of short cilia, either continuous, or interrupted posteriorly. The caudal is really a group of dorsal setae according to Maupas; see A. Z. Expt. (2), i. p. 535 et seqq, and Saville Kent's Infusoria, fig. I, p. 760.

2 For the limitations of the term Peritricha, see Geza Entz, Mitth. Zool. Stat. Naples, v. p. 407; Id. Z. W. Z. xxxviii. p. 183 et seqq.; Biitschli, M. J. xi. p. 553. The terms 'right' and 'left hand' applied to the adoral band of cilia mean the animal's right or left, the mouth being considered as ventral in position. For the paroral cilia of Tintinnodea, see Entz, op. cit. supra, Naples, pp. 395-6; for the peristome of the Hypotrichans, Saville Kent's Infusoria, fig. 2, pp. 760-1, and Biol. Centralblatt. iii. p. 236.

3 Actinobolus, Entz, Z. W. Z. xxxviii. p. 167; Mesodinium, Id. op. cit. supra, Naples, p. 308.

The protoplasm is either similar throughout, or it is divisible into an exoplasm (cortex) and endoplasm (medulla). In the former case it is variably granular, its movements slow as in Oxytrichidae and Euplotidae, and it is sometimes filled with non-contractile vacuoles1. When there is an exoplasm the transition from it to the more fluid endoplasm is gradual or abrupt. The granular character of the endoplasm depends much on the state of nutrition: the granules are albuminous, fatty, composed of glycogen, sometimes doubly refractile, occasionally distinctly,crystalline and apparently formed of a urate. A mouth or cytostome with an oral tube, and an anal spot or cytopyge, are absent only in the endoparasitic Opalinidae. The former may be terminal and anterior, even mounted on a protrusible eminence, e. g. Actinobolus, usually on one surface, hence the ventral, and frequently in connection with a peristome (supra). The cuticle lines the oral tube, which may have some extent. In the fixed Peritricha, e.g. Vorticella, there is a vestibule common to the oral tube, the anal spot and the duct of the contractile vacuole.

The oral tube may be armed with cuticular stylets, protrusible (Prorodon, Chilodon), or even discharged upon the prey (the Peritrichan Didinium): or with more complex apparatus (Dysteria armatd). In the fixed Peritricha a long vibratory seta, the optical expression according to some authorities of an undulatory membrane, projects from the vestibule. Food is conveyed into the oral tube by suction in carnivorous species, by the adoral band of cilia, which is prolonged into it in other instances, and it is then lodged in a food-vacuole. The anal spot is as a rule only visible at the moment of discharge, but in the parasitic Holotrichan Nyctotkerus, and perhaps some others, there is an anal tube with cuticular lining. The spot is terminal or subterminal, lateral in Stentor and Folliculina, in the vestibule of Vorti-cella and its congeners 2.

The protoplasm is generally colourless. It may be tinted by the food, e.g. Oscillatorians in Nassula, by a diffused pigment, such as the blue Stentorin of Stentor coeruleus, which yields a special absorption spectrum, the sea-green of St. multiformis, the blackish of St. niger, and dark-green of Folliculina, the yellow or brown of many species; or by pigment particles, such as the crimson globules of Holosticha flavo-rubra, the crimson granules of St. igneus, mixed with chlorophyl bodies which occur in that Heterotrichan, in St. polymorphic; Ophrydium versatile, and some others. Certain of the green-coloured forms, e.g. the last named, occur also colourless, and it has been supposed that the chlorophyl bodies are really symbiotic algae3. The exoplasm when differentiated is always eminently contractile. Distinct muscular fibrils are sometimes met with, and have been best worked out in Stentor and Vorticellina. In the former they are visible as fine longitudinal lines beneath the clear bands which bear the cilia of the body: the peristome has a few circular and radial lines beneath the adoral membranellae.

In the Vorticellina the fibrils spread from the apex of the body and are inserted into a thickened cuticular ring, the spot where the posterior ciliary wreath developes when the animal becomes free. Thence they extend to the edge of the peristome, which has its own fibril-system. They are continued in the opposite direction down the pedicle in Vorticella, Zoothamnium, and Carchesium as a spiral band. In the last-named the band is interrupted at each bifurcation of the pedicle. Trichocysts are lodged in the exoplasm of a few genera, for the most part Holotrichan. They are minute, fusiform bodies: when discharged they bear a fine thread attached to one end, or they are converted into straight filaments, pointed at each end, one end however furnished with a minute appendage1.

1Trachelitis ovum (Holotricha); Bursaria truncatella, Stentor coeruleus (Heterotricha); Loxodes rostrum, Condylostoma patens (Hypotricha).

2 On the supposed intestinal tube of Didinium, see Maupas, A. Z. Expt. (2) i. p. 597.

3 See pp. 242-5. Add to the authorities there quoted, Engelmann, Pfliiger's Archiv fun

There is a nucleus (endoplast) accompanied by one or more paranuclei (nucleoli s. endoplastules). It lies superficially as a rule, but is said to rotate in some instances with the endoplasm. It may be round or ovate, band-like, and then sometimes moniliform, i. e. beaded, the segments being connected by a delicate thread. There is a nuclear membrane, and contents variously disposed in different Infusoria. The paranuclei appear to be diminutive nuclei, and either lie near the nucleus or are apposed to it. Their number varies, indeed is inconstant in the same species. There is always one, but there may be more, e. g. 3-4 in Spirochona gemmipara, 28 in Stentor Roeselii. When the nucleus is beaded, a paranucleus may correspond to each segment, or the number of paranuclei may exceed or fall short of that of the segments2. In shape a paranucleus is globular; it has a membrane. There are some interesting variations from the typical structure. Loxodes rostrum and Holosticha Lacazei (Hypotrichd) have a number of nuclei with paranuclei, apposed one to each nucleus in the former, scattered in the latter.