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.
The eggs of Cypris ([Pl. IX.] fig. 35) are often found in glasses of water containing the animals. They are rounded or oblong, of a red colour, glued together by an amorphous jelly, and adherent to pieces of stick or the sides of the glass. They are enclosed in a thick shell, which exhibits a cellular appearance in the surface view, and is striated in the side view; so that the structure of the shell is prismatic, as in that of the oyster. When the eggs escape from the shell, they present the appearance represented in fig. 35 a, the body of the young animal being enclosed in a transparent envelope, one end of which forms a blunt protrusion; there is also a separate slender process enclosing the superior antennæ. After a time, the envelope is cast off (fig. 35 b), when the animal begins its active stage of life. The cast-off envelopes (fig. 35 c), with the protruded portions wrinkled, are often found in the sediment of water containing the animals. The structure of these ova is that of what are called winter ova, which agree with the resting-spores of the lower plants or the Algæ.
Cy´clops quadricor´nis ([Pl. IX.] fig. 30) is another common species. In this the body is closely surrounded by the jointed shell, as in a lobster. The superior antennæ (a) are very long and many-jointed, each joint having short bristles arising from it, while the inferior antennæ are short and four-jointed. There is no separate head, this being united to or consolidated with the first joint of the thor´ax or chest, the head and thorax together comprising four joints. The remaining joints enclose the belly, or abdómen, which has the appearance of a tail; but the tail is constituted by the two last parallel pieces, which are furnished with fine feathery filaments.
The female is most commonly met with, and is easily known by having the egg-pouch, or ovary (o) external on each side, and filled with eggs or ova. The little Cyclops is readily recognized by its form and jerking motion through the water.
Daph´nia púlex ([Pl. IX.] fig. 31) is a very common Entomostracan, and is very well adapted to illustrate the structure, on account of its size and transparence. In this animal the body is loosely connected with a bivalve shell, which, on careful examination, is seen to be reticulated or marked with net-like lines. The superior antennæ (a) are very small, placed under a small beak, and have at the end a minute tuft of hairs. The inferior antennæ (b) resemble arms, being large and branched; and by means of them the animal rows itself through the water. The structure of the eye is curious, consisting of a number of round lenses aggregated together, the fine muscular threads by which it is moved being easily distinguished with a high power. The legs are flattened, and furnished with elegant feathery sétæ (seta, a bristle), serving as gills or branchiæ. They are constantly in motion, fanning the water so as to change incessantly the portion with which they are in contact. About the middle of the back is placed the little transparent heart, with its colourless blood, which may be distinctly seen beating, or contracting and dilating, in the living animal; and between the back of the animal and the shell are seen the ova, which remain there until they are hatched.
The genera and species of the Entomostraca are very numerous. Those mentioned above will serve to illustrate the general structure of the order. To distinguish the man´dibles (mandib´ula, a jaw) or proper jaws, the foot-jaws, and the branchial legs, the animals must be dissected in water with the mounted needles. The very delicate feathery filaments of the branchiæ may be best observed when these organs are dried on a slide.
The Entromostaca may be kept alive in a jar of water with water-plants for a long period. They may be removed from the water for examination by the dipping-tube, and are best observed in a live-box.