animal after a few changes into a region where the solution is strong enough to kill it. This “motile reaction,” first noted by H. S. Jennings, is the explanation of the general reactions of minute swimming animals to most stimuli of whatever character, including light; the practical working out is, as he terms it, a method of “trial and error.” The action, however, of a current of electricity is distinctly and immediately directive; but such a stimulus is not to be found in nature. The motile reaction in the Hypotrichaceae which crawl or dart in a straight line is somewhat different, the swerve being a simple turn to the right hand—i.e. away from the mouth.
Parasitism in the Infusoria is by no means so important as among Flagellates. Ichthyophthirius alone causes epidemics among Fishes, and Balantidium coli has been observed in intestinal disease in Man. The Isotricheae, among Aspirotrichaceae and the Ophryoscolecidae among Heterotrichaceae are found in abundance in the stomachs of Ruminants, and are believed to play a part in the digestion of cellulose, and thus to be rather commensals than parasites. A large number of attached species are epizoic commensals, some very indifferent in choice of their host, others particular not only in the species they infest, but also in the special organs to which they adhere. This is notably the case with the shelled Peritrichaceae. Lichnophora and Trichodina (fig. iii. 8, 9) among Peritrichaceae are capable of locomotion by their permanent posterior wreath or of attaching themselves by the sucker which surrounds it; Kerona polyporum glides habitually over the body of Hydra, as does Trichodina pediculus.
Several Suctoria are endoparasitic in Ciliata, and their occurrence led to the view that they represented stages in the life-history of these. Again, we find in the endosarc of certain Ciliates green nucleated cells, which have a cellulose envelope and multiply by fission inside or outside the animal. They are symbiotic Algae, or possibly the resting state of a Chlamydomonadine Flagellate (Carteria?), and have received the name Zoochlorella. They are of constant occurrence in Paramecium bursaria, frequent in Stentor polymorphus and S. igneus, and Ophrydium versatile, and a few other species, which become infected by swallowing them.
Order I.—Section A.—Gymnostomaceae. Mouth habitually closed; swallowing an active process; cilia (or membranelles) uniform, usually distributed evenly over the body; form variable, sometimes of circular transverse section.
Section B.—Trichostomata. Mouth permanently open against the endosarc, provided with 1 or 2 undulating membranes often prolonged into an inturned pharynx; ingestion by action of oral ciliary apparatus.
Order 2.—Subsection (a).—Aspirotrichaceae. Cilia nearly uniform, not associated with cirrhi or membranelles, nor forming a peristomial wreath. Form usually flattened, mouth unilateral. (N.B.—Orders 1, 2 are sometimes united into the single order Holotrichaceae.)
Subsection (b).—Spirotricha. Wreath of distinct membranelles—or of cilia fused at the base—enclosing a peristomial area and leading into the mouth.
§§ i.—Wreath of separate membranelles.
Order 3.—Heterotrichaceae; body covered with fine uniform cilia, usually circular in transverse section.
Order 4.—Oligotrichaceae; body covering partial or wholly absent; transverse section usually circular.
Order 5.—Hypotrichaceae; body flattened; body cilia represented chiefly by stiff cirrhi in ventral rows, and fine motionless dorsal sensory hairs.
Order 6.—§§ ii.—Peritrichaceae. Peristomial ciliary wreath, spiral, of cilia united at the base; posterior wreath circular of long membranelles; body circular in section, cylindrical, taper, or bell-shaped.
Illustrative Genera (selected).
1. Gymnostomaceae. (a) Ciliation general or not confined to one surface. Coleps Ehr., with pellicle locally hardened into mailed plates; Trachelocerca Ehr.; Prorodon Ehr. (fig. i. 6, 7); Trachelius Ehr., with branching endosarc (fig. i. 8); Lacrymaria Ehr. (fig. i. 5), body produced into a long neck with terminal mouth surrounded by offensive trichocysts; Dileptus Duj., of similar form, but anterior process, blind, preoral; Ichthyophthirius Fouquet (fig. i. 9-12), cilia represented by two girdles of membranellae; Didinium St. (fig. i. 13), cilia in tufts, surface with numerous tentacles each with a strong terminal trichocyst; Actinobolus Stein, body with one adoral tentacle; Ileonema Stokes. (b) Cilia confined to dorsal surface. Chilodon Ehr.; Loxodes Ehr., body flattened, ciliated on one side only, endosarc as in Trachelius; Dysteria Huxley, with the dorsal surface hardened and hinged along the median line into a bivalve shell, ciliated only on ventral surface, with a protrusible foot-like process, and a complex pharyngeal armature. (c) Cilia restricted to a single equatorial girdle, strong (probably membranelles); Mesodinium, mouth 4-lobed.
2. Aspirotrichaceae. Paramecium Hill (fig. ii. 1-3); Ophryoglena Ehr.; Colpoda O. F. Müller; Colpidium St.; Lembus Cohn, with posterior strong cilium for springing; Leucophrys St.; Urocentrum Nitsch, bare, with polar and equatorial zones and a posterior tuft of long cilia; Opalinopsis Foetlinger (fig. i. 1, 2); Anoplophyra St. (fig. i. 3, 4). (The last two parasitic mouthless genera are placed here doubtfully.)
3. Heterotrichaceae. (a) Wreath spiral; Stentor Oken. (fig. iii. 2), oval when free, trumpet-shaped when attached by pseudopods at apex, and then often secreting a gelatinous tube; Blepharisma Perty, sometimes parasitic in Heliozoa; Spirostomum Ehr., cylindrical, up to 1′ in length; (b) Wreath straight, often oblique; Nyctotherus Leidy, parasitic anus always visible; Balantidium Cl. and L., parasitic (B. coli in man); Bursaria, O.F.M., hollowed into an oval pouch, with the wreath inside.
4. Oligotrichaeceae. Tintinnus Schranck (fig. iii. 3); Trichodinopsis Cl. and L.; Codonella Haeck. (fig. iii. 5); Strombidium Cl. and L. (fig. iii. 4), including Torquatella Lank. (fig. iii. 6, 7), according to Bütschli; Halteria Duj., with an equatorial girdle of stiff bristle-like cilia; Caenomorpha Perty (fig. iii. 23, 24); Ophryoscolex St., with straight digestive cavity, and visible anus, parasitic in Ruminants.
5. Hypotrichaceae. Stylonychia Ehr.; Oxytricha Ehr.; Euplotes Ehr. (fig. i. 14, 15); Kerona Ehr. (epizoic on Hydra).
6. Peritrichaceae. 1. Peristomial wreath projecting when expanded above a circular contractile collar-like rim.
(a) Fam. Urceolaridae: posterior wreath permanently present around sucker-like base. Trichodina Ehr. (fig. iii. 8, 9), epizoic on Hydra; Lichnophora Cl. and L.; Cyclochaeta Hatchett Jackson; Gerda Cl. and L.; Scyphidia Duj.
(b) Fam. Vorticellidae = Bell Animalcules: posterior wreath temporarily present, shed after fixation.
Subfam. 1. Vorticellinae animals naked. (i.) Solitary; Vorticella Linn. (fig. iii. 11-17), stalk hollow with spiral muscle; Pyxidium S. Kent, stalk non-contractile. (ii.) Forming colonies by budding on a branched stalk: Carchesium Ehr., hollow branches and muscles discontinuous; Zoothamnium. Ehr., branched hollow stem and muscle continuous through colony; Epistylis Ehr., stalk rigid—(the animal body in these three genera has the same characters as Vorticella)—Campanella Goldf., stalked like Epistylis, wreath of many turns (nematocysts sometimes present) (fig. iii. 19); Opercularia, stalk of Epistylis, disk supporting wreath obconical, collar very high (fig. iii. 20).
Subfam. 2. Vaginicolinae; body enclosed in a firm theca: Vaginicola Lam., shell simple, sessile; Thuricola St. Wright, shell sessile, with a valve opening inwards (fig. iii. 25-26); Cothurnia Ehr., shell stalked, simple; Pyxicola S. Kent, shell stalked, closed by an infraperistomial opercular thickening on the body (fig. iii. 21-22).
Subfam. 3. Shells gelatinous; those of the colony aggregated into a floating spheroidal mass several inches in diameter Ophrydium Bory, O. versatile contains Zoochlorella, which secretes oxygen, and the gas-bubbles float the colonies like green lumps of jelly.
2. Peristomial wreath, not protrusible, surrounded by a very high usually spiral collar.
Fam. Spirochonina. Spirochona St. (fig. iii. 10); Kentrochona Rompel; both genera epizoic on gills, &c., of small Crustacea.
Suctoria.—These are distinguished from Ciliata by their possession of hollow tentacles (one only in Rhyncheta, fig. viii. 1, and Urnula) through which they ingest food, and by not possessing cilia, except in the young stage. Fission approximately equal is very rare. Usually it is unequal, or if nearly equal one of the halves remains attached, and the other, as an embryo or gemmule, develops cilia and swims off to attach itself elsewhere; Sphaerophrya (fig. viii. 2-6) alone, often occurring as an endoparasite in Ciliata, may be free, tentaculate and unattached.
The ectosarc is usually provided with a firm pellicle which shows a peculiar radiate “milling” in optical section, so fine that its true nature is difficult to make out; it may be due to radial rods, regularly imbedded, or may be the expression of radial vacuoles. The tentacles vary in many respects, but are always retractile. They are tubes covered by an extension of the pellicle; this is invaginated into the body round the base of the tentacle as a sheath, and then evaginated to form the outer layer of the tentacle itself, over which it is frequently raised into a spiral ridge, which may be traced down into the part sunk and ensheathed within the body: in Choanophrya, where the tentacles are largest, the pellicle is further continued into the interior of the tentacle. The tentacles are always pierced by a central canal opening at the apex, which may be (1) enlarged into a terminal capitate sucker, (2) slightly flared, (3) truncate and closed in the resting state to become widely opened into a funnel, or (4) pointed. The tentacles are always capable of being waved from side to side, or turned in a definite direction for the reception or prehension of food; in Rhyncheta, the movements of the long single tentacle recall those of an elephant’s trunk, only they are more extensive and more varied. In the majority of cases the food consists of Ciliata; and the contents of the prey may be seen passing down the canal of the sucker beyond where it becomes free from the general surface. In Choanophrya the food appears to consist of the débris of the prey of the carnivorous host (Cyclops), which is sucked into the wide funnel-shaped mouths of the tentacles—by what mechanism is