Lastly, we have an exquisite group of organisms related to Vorticella, but possessing a transparent envelope, the forms of which are most varied, but always graceful. _Vaginicola_ (Fig. 17) is a good example of this, and _Cothurnia_ (Fig. 20) still more so. Many of these organisms, too, are furnished with a plate, attached either to the head or to the body, which plate, when they withdraw into their cases, closes the latter perfectly, as in the case of the exquisite _Pyxicola_ (Fig. 21). A very interesting but singularly obtrusive organism is the _Stylonychia_ (Figs. 22, 23). How often has it happened to us to have an interesting object nicely in the field of view, and then to have it knocked out of sight by the blundering incursion of this burly fellow, who runs so rapidly by means of his “styles” that he gives nothing time to get out of the way. He is of interest to us, however, as the representative of a class in which the body is not ciliated, or very partially and slightly so, usually round the mouth. We have frequently found Stylonychia, in company with Vorticella and _Paramœcium_ (Plate IX. Fig. 6), in the water in which flowers have been standing for a few days; sometimes the numbers are so great as to make the water quite milky. One more form must conclude this short sketch of the great Infusorial family. It is the _Acineta_ (Fig. 24), which, attached by its foot-stalk, and devoid of cilia, patiently waits, with outspread arms, to receive and embrace smaller members of the family as they dance merrily about. Alas! its embrace is as fatal as that of the image of the Virgin which bore beneath its robe spikes and daggers, for the victim struggles vainly to escape, and the nourishment from its body is rapidly absorbed. And here we take our leave of a group which, simple as is the construction of the animals which it includes (for every one, great and small alike, is composed of a single cell), is yet full of beauty and interest. He who wishes to pursue the matter further will find in Saville Kent’s _Manual of the Infusoria_ a perfect mine of information, to which we gladly acknowledge our indebtedness, both now and in time past. CHAPTER IX Fresh-water Worms--Planarians--Hydra--Polyzoa--Rotifers--Chætonotus-- Water-Bears. The fresh-water worms form a large and well-defined group, and a few words regarding them may be useful. They are very common, and very difficult to find information about, most of the work relating to them having been done in Germany. At the same time, they are so highly organised and so transparent that the process of their life-history may be easily followed.