principal point of the eyepiece, and B that of the objective. S is the anterior, and D the posterior, principal points of the entire microscope. T is the anterior, and F the posterior, focal points of the microscope. ST, D= F, focal length of microscope. It will be seen that the foci lie inside the principal points. BA is the optical tube-length." THE Rotifers and where to find them. HE study of Natural History is doubtless destined to replace in a great measure that love of sport, to which in some degree it has an affinity, which has become so ingrained in human nature. Our prehistoric ancestors hunted, amongst other animals, the mammoth, woolly-haired rhinoceros, and the large Irish elk. The ancient Britons, in like manner, pursued the primitive ox, the wild boar, the bear, and the wolf. But at the present day in these isles sportsmen have to be content with tame deer, the fox, the hare, occasionally a badger, the rat, pheasants, partridges, ducks, and other small (sometimes very small) fry. Even these (except, perhaps, the rat) have to be preserved, in order that they may appease the lust of slaughter of the fin de siècle Nimrod. Many of us now satisfy instincts, which are essentially of the same origin, by the study of one or other of the diverse forms of life. And it is my present purpose to draw especial attention to a group of organisms which, although extremely minute, are of surpassing beauty-namely, the Rotifera. Indeed, we cannot conceive a branch of natural history more interesting than the study of this highly-specialised group of animals. No book, not even the historic and beautiful monograph on the Rotifera by Hudson and Gosse, can give the knowledge and delight which may so easily be derived by observing the creatures themselves in a living state. Rotifers are small aquatic animals, varying from 1/8th to 1/500th of an inch in length. They derive their name from the wheel-like appearance produced by the motion of the fine circlets. of cilia which arise on the front of their heads, it being by the vibratile action of these cilia that they swim and disport themselves in the water. They possess a simple stomach and intestine ; jaws to crush their food; and muscles, which are sometimes striated, pass through the body cavity; they have a well-developed vascular system; and a brain or nervous ganglion, from which nerve-threads radiate to the sensory organs. The creatures are dicecious, producing their young from two kinds of ova male and female. The Rotifera are divided into four orders, which are again subdivided into twenty-one families, each family containing from one to twelve genera, and each genus consisting of from one to fifty species; nearly six hundred species are known at present, and observers are constantly adding to the number. Rotifers are to be found in more or less abundance in almost every pool of water; but the order Rhizota (that is, those fixed by the foot), which includes the genera Floscularia, Stephanoceros, Melicerta, Ecistes, Lacinularia, Limnias, Cephalosiphon, etc., are to be found only in waters where healthy aquatic plants grow, such as in lakes, ponds, and marsh-pools. Waters which are undisturbed are their favourite haunts. Floscularia is a wide-spread genus, and a more beautiful or interesting creature it would be hard to find. As the finest examples of the genus can only be obtained at a little distance from the edge of a lake or pond, it is therefore necessary to procure a boat for the purpose of dredging up the plants on which they abide. If a boat is not to be procured, the collector must furnish himself with a line, having a sinker and hook attached, which must be thrown as far as possible into the water, and by that means the weed can be drawn out. The collector should also provide himself with a few wide-mouthed clear-glass bottles, so that he may be able to examine the weed and water with a pocket-lens, since to carry away a bottle with weed and water from the side of a pond without examining it is by no means a good method of collecting. The Melicertians can be easily discerned in this way; but the Floscules are not so easily detected, as their bodies and tubes are in many instances so extremely transparent that it is difficult to make them out, even with a one-inch objective under the microscope, except by oblique illumination. The most effective way of discerning these and similar forms in the collecting-bottle, is to hold the bottle in the left hand between the eye and the light, spreading two fingers so as to obstruct the direct rays; the pocket-lens is held in the right hand, and the light being transmitted through the contents of the bottle at an oblique angle, Floscules and other transparent forms may be readily seen, and the collector may estimate whether the material is worth carrying home. If the dark-tubed Melicertians are detected, in nine cases out of ten there will be found associated with them one or more species of Floscules perched on the leaves and stems of the same weed. In all collections of water-weed procured in this way, there will also be many free-swimming Rotifera of various species, such as Notommata, Pterodina, Euchlanis, Brachionus, etc. The Philodinca, with few exceptions, must be sought for at the bottom of ponds, ditches, and marsh-pools, amongst moss and decayed vegetable débris. I have found the Philodina erythrophthalma perched as close as they could stand on the tubes of Alcyonella fungosa, Plumatella repens, and other fresh-water polyzoa, and even on the shells of Limnæa stagnalis and other freshwater molluscs; whilst on the other hand Philodina roseola loves to dwell in stagnant pools, water-butts, house-gutters, and even newly-filled rain-water pools. The Synchetada are chiefly found in clear lakes and ponds, and in the open sea. But there are exceptions to general rules even in Rotifers, for the largest and finest examples of S. pectinata and S. tremula I have ever met with were found in a peat-bog in the west of Ireland, where the water was of a deep port-wine colour. The Synchatada are vigorous swimmers, and are the swallows of the waters. S. longipes is, I think, the swiftest, ever dashing on, whirling round and round, never still for an instant from the moment it leaves the egg till its death, not even stopping to take a meal, but devouring its prey as it dashes along. It is wholly carnivorous, its food consisting of living rotifers when it can get them. It exhibits a preference for, and loves to feed on, its second cousin, Polyarthra phalyptera, of which there are commonly numbers in the same habitats. It seizes its victim invariably by the head, swims through the water with it in its mouth without the least diminution of speed, all the time sucking the internal juices. of its victim, then dropping the empty skin to seize the next Polyarthra it meets. It is ever ready for a meal, and appears to enjoy a very merry life, although a short one. Its whole lifetime from youth to old age does not exceed sixty-four hours. The Hydatinida are to be found not far from farmyard pools where cattle and horses drink, pools not far from manure heaps being the favourite haunts of Hydatina senta, associated with Euglena, on which the H. senta freely live. Callidina elegans and C. bidens are frequently found in the same pools. The members of Conochilus volvox are objects of great beauty. They form a lovely sphere of living creatures of from ten to one hundred individuals, their feet fixed in a common centre and radiating outwards, each bearing its flashing corona of whirling cilia, pretty red-orange tinted eyes, and constantly moving jaws. The action of their ciliary wreath imparts to the group a gentle rolling motion. Fortunately C. volvox is not a rare object, being found in ponds and marsh-pools, and can be collected by fastening at bottle at right angles to the end of a walking-stick, and dipping the bottle at various depths, sometimes on the surface or from one to three feet under the surface of the water, close to the roots of rushes. It can be readily discerned in the bottle, in the form of whitish discs when held between the eye and the light, and may be kept living in a bottle for a few weeks if one or two stalks of the decayed rush be put in the water along with it. C. unicornis is also a lovely object when found in its normal state in spherical colonies, but is more delicate than C. volvox, and with a slight agitation the globular colony is prone to break up into single individuals, encased in a transparent gelatinous envelope or tube. It is only found singly in clear, running streams, the spherical colonies being found, however, in lakes and reservoirs. The species was discovered by Mr. C. F. Rousselet a few years ago. It is, however, widely distributed. I have found it in colonies of from twenty to one hundred individuals in lakes both in Scotland and Ireland. The Asplanchnada is not a large family, consisting of but three genera, namely-Asplanchna, Asplanchnopus, and Sacculus; and although not possessing the beauty of Conochilus or Stephanoceros, they are of the greatest delicacy, especially Asplanchna and Asplanchnopus, appearing like bubbles of the purest glass; both being of large size, they give unusual facilities for the observation and study of their internal organs. They are carnivorous, their food consisting of large infusorians, rotifers, even the smaller forms of Entomostracans, are devoured by them. Asplanchna is viviparous, but Asplanchnopus deposits its eggs on weeds or on the mud at the bottom, and are to be found in shallow ponds, marshes, and ditches. The Notommatida is a large family of eleven or twelve genera, each including a number of species; many of them are creatures of the greatest interest. Some of the species have strange dwelling-places. Proales parasita may be found swimming about within the beautiful spheres of Volvox globator, from which the creatures derive their sustenance and in which they deposit their eggs. The adult Proales parasita, after it has deposited its eggs in the interior of the Volvox globator, eats its way out, leaving its eggs (which are generally four in number) to hatch. It then swims through the water until it meets a suitable Volvox in which to make a new home, effecting an entrance by eating a hole in the cuticle large enough to admit it, and there it lives in ease and comfort, with food enough within its revolving domicile to sustain it for the rest of its life. The largest individuals of the family Notommatida are those of the genus Copeus, and although no species of that genus can be called pretty in form, what they are wanting in symmetry and grace they gain in interest, for their bodies are fairly transparent, permitting the internal organs to be easily seen and studied. Their motions when not swimming are slow, rendering to the observer great facilities for watching them feeding. Their food consists of Diatoms and Desmids. The smaller forms are swallowed whole, but in the case of the larger forms-such as the Pinnularia and Closterium-- they nibble at an end of a frustule until a breach is made. Then the endochrome of the cell may be observed to pour down the creature's oesophagus into its stomach. Copeus is only to |