_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. ARACH´NIDA (ἁρἁχνη, spider) is the Class of spiders, scorpions, and mites. _Aranéida._--This Order contains the more highly developed forms of the Class, among which are the common spiders of houses and gardens; and some of their structures are very curious and interesting. The head of spiders is united or fused with the thorax, forming one piece, which is called the _ceph´alothorax_ (κεφαλἠ, head, θὡραξ, chest). The claw-jaws, or _man´dibles_, are terminated by a curved and pointed claw, with which the spiders hold their prey. It is traversed by a slender canal, containing a slender tube or duct leading from a poison-gland, and opening near its point; and when the insect prey is transfixed by the mandible, the poison is pressed out and enters the wound.