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.