The polypidoms of the Bryozoa form interesting microscopic objects, the cells being furnished with variously arranged spines and punctures or dots. In some the cells are erect and arranged in rows upon the branches of a plant-like stem, while in others they are scattered irregularly over a creeping filament. For examination they should be prepared by maceration in fresh water, and drying between glass plates or sheets of paper, and either viewed as opake objects or, after soaking in turpentine and mounting in balsam, as transparent objects. It may be remarked that the name Bryozoa for this class of Mollusca, which was thoroughly established, has recently been changed in this country to Polyzóa (πολὐς, many, ξῶον, animal), and that the name of polypidom has been altered to polyzóary. CHAPTER XI. ARTICULATA (ARTIC´ULUS, A JOINT). The animals belonging to this subkingdom are specially distinguished by the body and limbs being jointed: as familiar instances, may be mentioned the lobster, the wood-louse, spiders, insects, and worms. Taking the class Crustacea, to which the two first animals belong, we find interesting microscopic forms in the subclass Entomos´traca (ἔντομον, insect; ὄστρακον, shell). ENTOMOSTRACA.--The animals contained in this Order are met with in every pool or pond, some of them inhabiting the sea. They are mostly minute, yet visible to the naked eye, forming specks swimming actively or leaping through the water; hence some of them have been called water-fleas. The body of the animal is protected by a shell or car´apace, which in some consists of a single piece (Pl. IX. fig. 30), while in others it consists of two similar parts or valves (fig. 31), in the latter case the joints of the body being indistinctly visible. The head is furnished with usually two projecting feelers or antennæ (_anten´na_, a sail-yard), one of which is uppermost or superior (Pl. IX. figs. 30, 31, 34 _a_), the other lowermost or inferior (figs. 30 and 34 _b_); and these are often used for swimming. The antennæ are jointed, and sometimes beautifully plúmose (_pluma_, a feather) or feathery, _i. e._ furnished with rows of long and very slender filaments. There are several pairs of jointed legs, some of which serve as jaws (foot-jaws), while others are finely filamentous to serve for swimming and as respiratory organs (branchial feet). The four species figured are very common. _Cypris tristriáta_ (Pl. IX. fig. 34) is found in ponds and ditches. The carapace is bivalve, or has two valves, which are convex and oval; and it is of a greenish colour, with three irregular dark stripes behind. The superior antennæ (_a_) are jointed and finely feathery, the inferior antennæ (_b_) having a tuft or pencil of fine filaments arising from their anterior margin. The eye is single. The animal swims steadily and freely through the water.