Rotifera

ROTIFERA (or Rotatoria), a well-defined class of aquatic animals of microscopic size, remarkable for the astonishing diversity of their forms, the vivacity and intelligence of their movements and the high level of their structural development. Being extremely transparent, the largest can hardly be detected by the unaided eye. In length they rarely exceed 1.7 mm. or -A in., ranging downwards to .o8 mm., and they probably average under .25 mm., with breadth and thickness very considerably less. In general rotifers are compact in body and symmetrical in structure. They are plentiful in most weedy ponds and boggy pools, and are also to be found in lakes and reservoirs, canals and rivers, ditches and runnels,—in short, in any collection of water containing suitable food. The great majority live in fresh-water, yet many are seldom met with except in water either brackish or alkaline, and some are restricted to sea-water. Many flourish in places only intermittently wet, such as among the stems of land growing mosses and liverworts. Some are parasitic within the cells of water-plants, or the bodies of other larger water-animals, living even in the gill-chambers of fresh-water crabs and cray fish. Others are literally "hangers-on" to the leaves and stems of water-weeds for the sake of a favourable position, or to the exte riors of water animals for the benefit of constant change of surroundings as their hosts move about. While they mostly live in waters of moderate temperature, they have been found established in hot springs. They can endure intense cold, being capable of reviving after being frozen in thick ice. Some appear only in the spring and summer, others carry on through the winter as well.

The duration of their individual lives has been little studied, but some species are known to live a few days only, while others survive for at least three months. They are greatly dependent upon their immediate environment and the quality, no less than the quantity, of the water inhabited. Sud den changes of temperature, or in the den sity of the water, or, perhaps, a decrease in the proportion of oxygen held in solution, are quickly fatal to them.

Whatever the variations of their outward form, an arched back and a flattened ven tral surface, two similar sides and a division of the body into head, trunk and foot, by shallow constrictions, can in general be readily distinguished, though the head is often merged into the trunk, and there is frequently no foot. The whole

may be maggot-shaped, slender or elongate, ovoid or squat. Especially diversified in form are those species in which the skin is hardened to become an armour-like covering (lorica) which may be much flattened or laterally compressed. While mostly smooth and hyaline, the surface may be shagreened, faceted, grooved or otherwise obscured. It may carry defensive spines, supplemented by numerous prickles. It may consist of one or of several pieces connected by yielding skin. In species without a lorica, the skin is generally smooth and flexible, but is sometimes tough and leathery, and may carry spines. The head, trunk and foot are often sub divided into smaller areas, segments or joints, by annular in foldings of the skin, frequently permitting the telescoping of one segment into that next to it.

Corona.

The collection of food and the swimming and gliding movements of rotifers are effected by the lashing action of numer ous cilia crowded upon a particular area of delicate skin close to or encircling the mouth, or fringing several fleshy lobes or discs protrusible from it. The whole area, including the mouth itself, as seen when the cilia are active, is called the corona, and there are many varieties of the organ, dif fering widely in the arrangement of the cilia, etc. All these may be assigned to two leading types, the external and the evertile.

In the main, the external type is character istic of the hunting rotifers, which go about, swimming or gliding, in search of their food. The mouth is generally a little below the centre of the convex front of the head and the ciliated area, sometimes extending over various prominences, may be mainly before or mostly behind it. In certain spe cies which only swim feebly by their ordi nary cilia, these are often supplemented by auricles, small evertile pouches, one on each side of the head, lined with more powerful cilia. When the pouches are everted, these stronger cilia drive the rotifer along at greatly augmented speed. Other species rely al most entirely on their auricles. Certain footless species possess, besides the corona, from two to twelve leaping spines, attached to the "shoulders," which enable them, in emergencies, suddenly to spring several times their own length. In Pedalia, these spines are replaced by six limbs, having flattened ends fringed with stiff bristles.