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 unknown. The endosarc is full of food-granules and reserve-granules (oil, colouring matter and proteid).