Common objects of the microscope

by J. G. Wood

Another group of worms is the Planarians, small leech-like worms, black, white, or brown, which are rarely absent from a gathering. The would-be investigator will find in them an abundant field for work, as they are but very imperfectly known or studied.

The great enemy of all these worms is the Hydra, a good idea of which may be formed from Plate IX. Fig. [13]. There are three species, all of which are fairly common. They capture their prey in exactly the same way as sea-anemones and the marine hydroid forms, so numerous and varied.

Nor must we omit to notice the exquisitely beautiful Polyzoa, such as Lophopus (Plate XIV. Fig. [4]), with its ciliated tentacles and transparent social home; Fredericella (Fig. [5]), with its graceful stems, and their still more graceful inhabitants; and the wonderful Cristatella, whose colonies form bodies which crawl over the stems of water plants. But for grace, beauty, and variety, the Rotifers assuredly outshine all their fellow inhabitants of our ponds and streams.

We can only take a few types, and of all these the most common is the common Rotifer (Plate IX. Fig. [10]). It is there shown in the act of swimming, but it can withdraw its “wheels” and creep like a leech, protruding its foot as it does so. It is distinguished by the two eye-spots on the proboscis from Philodina, in which they are on the breast, and Callidina, which has none. When at ease in its mind, the animal protrudes its wheels, and by their action draws in particles of food, these passing down to the incessantly moving jaws, which act like a mill and crush the food before it passes on to be digested. The movement of the jaws may even be seen in the young Rotifer whilst still in the egg within the body of the parent, and as the egg reaches its full development other eggs again are visible within it, so that we may have three generations in one individual. The males of most of the Rotifera are unknown. Those that are known are very lowly organised, having only the ciliary wreath and the reproductive organs, and are only found at certain seasons of the year. For the remainder of the time parthenogenesis is the rule, just as among the Aphides. We select a few individuals for illustration as types. Those who wish to pursue this study further must be referred to the monumental work of Hudson and Gosse.

The common Rotifer, already referred to, may be taken as the type of the Bdelloida, or leech-like class, so called from their mode of “looping” themselves along. The group is a comparatively small one in comparison with the next, the Ploïma, or free-swimmers. We can only select from the vast variety a few species, first of those classed as illoricated, from their being without a loríca, or case, and then of the loricated, which possess it. A very large and common form is Hydátina (Plate XIV. Fig. [7]), which lives by choice in the reddish pools of water found often by the roadside. It shows the whole organisation of the class magnificently; the ciliary wreath on the head, with the striped muscles which draw the latter back, the powerful jaws, the digestive canal with its crop and intestine, the ovary with the developing eggs, the water-vascular system with the curious vibratile tags, and finally, the cloaca, which receives the waste of the body and expels it at intervals.

Another form, also common, especially in clear water, is Synchæta (Fig. [8]), very much like a kite or peg-top in shape, which has the power of attaching itself by a glutinous thread, and spinning round at a tremendous rate. Then there is the gigantic Asplanchna (Fig. [9]), which has no opening below, so that the waste must be discharged by the mouth; and curious Sacculus, which gorges itself with chlorophyll until it looks like a green bag with a string round it, but clear and sparkling. Of the Notommatæ there is a whole host, but we can only mention the beautiful N. Aurita (Fig. [10]), with an eye of a beautiful violet colour, composed of several spherules massed together, and two curious ear-like processes on the head, from which it takes its name. Some of the Ploïma have powers of leaping which must be noticed. The Triarthra (Fig. [11]) has three arms, or what we may call such, which it can stretch out suddenly and leap to a considerable distance, whilst in Polyarthra the arms become a whole cluster of broad saw-like bristles.

We pass on to note a few species of the mail-clad or loricated Rotifers, chief among which the great Euchlanis (Fig. [12]), a noble-looking fellow, calls for our attention, his great size rendering him easily visible to the naked eye. It is difficult to avoid using the masculine gender, but, of course, all those figured and described are of the gentler sex. Salpina, too (Fig. [14]), with its box-like lorica, armed with spines at each of the upper angles, and having three below, is quite easily recognised, and very common. Brachionus (Fig. [13]) has a shield-shaped case, well furnished with spines, symmetrically arranged at the top, and an opening below for the flexible wrinkled tail, like the trunk of an elephant. Pterodina (Fig. [15]) has a similar tail, but a round case, and the head is much more like that of the common Rotifer when extended. Anuræa (Fig. [16]), on the other hand, has no tail, and its case is shaped like a butcher’s tray, with a handle at each corner. Dinocharis (Fig. [17]) has a roof-like case, with long spines on the root of the tail, and a forked stiff foot. Noteus (Fig. [18]) is much like Pterodina, except in its foot, which more nearly resembles that of Dinocharis.

The list might be indefinitely extended, but sufficient has probably been said to enable the tyro to find his bearings in this large, beautiful, and interesting class.

We pass on to notice in conclusion two or three of the fixed forms, of which a beautiful example is the Melicerta ringens (Plate IX. Fig. [7]), whose building operations have a never-ending charm. Particles of débris are accumulated in a curious little cavity in the chin, in which they are whirled round, and mixed with a secretion which binds them together, and when a brick is made the head is bent down and the brick applied to the desired spot with mathematical regularity. By supplying fine particles of innocuous colouring matters, the Melicerta may be made to build a variegated case. The most remarkable specimen known is the one figured in Hudson and Gosse’s work, which was found by the present writer in a specimen of water from which he had already obtained five-and-twenty species of various kinds of Rotifer; the water was collected by an inexperienced person, and there was only a pint of it. It had, moreover, been kept for three weeks, and the moral of that is, to preserve one’s gatherings, and keep an aquarium into which they may be poured when done with for the moment. New forms will often develop with startling rapidity, their eggs having been present in the original gathering. The young form of Melicerta, shown in Plate XIV. Fig. [20], is strangely unlike its mother, and much more nearly resembles its father.

Another group of extreme beauty is the Flosculariæ (Fig. [19]), several species of which are very common. They will be easily known by their appearance, which resembles a shaving brush when closed; whilst, when opening, the shaving brush resembles a cloud of delicate shimmering threads, which at last stand out straight, radiating all round the head of the creature, and forming the trap by means of which it catches its prey. Finally, there is the lovely Stephanoceros (not, unfortunately, very common), with its five symmetrically placed and gracefully curved arms, perhaps the most lovely of all Rotifers, with its exquisitely transparent body, sparkling with masses of green and golden brown. He who finds this has a treasure indeed, and will be encouraged to prosecute his studies in this “Fairyland of Microscopy.”