cucumber, where it is connected with gill-like appendages about the mouth. The intestinal worms and other annelids possess this style of 
Fig. 7.—Anatomy of a Bivalve Mollusk (Mya arenaria). The left, valve and mantle-lobe and half the siphons are removed, ss, respiratory siphons, the currents; a a', adductor muscles; b, gills; h, heart; o, mouth, surrounded by (p) labial palpi; f, foot; v, anus; m, cut edge of the mantle. (After Woodward.) breathing-organs. Later we shall find in the insects a similar and perhaps homologous system for conveying air.
Most aquatic invertebrate animals which have a true circulating fluid send this to expansions of the skin to be there oxygenated by the surrounding water. This is termed. "branchial" respiration. The branchiæ or gills may be wholly exposed and external to the body, or may be contained in suitable cavities. They may develop on any part of the body; and they exhibit a great variety in many respects. With this system is frequently found some special accessory apparatus for producing and regulating a flow of water over the gills.
The sea-worms have a great variety of external gills developed immediately from the skin. Renewal of water is secured by cilia which cover the body or even the gills. These branchiæ are finely shown in the common Serpula, where they form a crown of scarlet plume-like about the mouth. The Arenicola, or sand-worm, has two rows of crimson rosettes along the sides of the body. The Eunice, Nereis, and sea-mouse, are other worms possessing beautiful arborescent fringes of branchiæ.
As the crustaceans, excepting a few of the arrows indicating the direction the lowest, are covered with a hard shell of dead matter, they can not respire through the skin, and hence of necessity require special organs. These are commonly leaf-like expansions, usually attached to the locomotive or other appendages; the purpose of such attachment being to secure rapid change of the surrounding water. In some groups, the entire limb becomes foliaceous and respiratory, so that it is literally correct to say that the fairy shrimp breathes with its legs. Other forms, including the sand-hopper, whale-louse, and fresh-water shrimp, breathe by vesicles or bladders attached to the limbs. The water-fleas, as Cypris and Cyclops, so common in our fresh waters, have their branchiæ attached to the jaws. Many forms, among which are the king-crab, or Limulus, and
