plied to the gill-chamber by the open mantle; but, by the contraction of the latter, the water is forcibly expelled through a tube termed the "funnel." This expulsion of water from the branchial chamber is their chief means of locomotion.
Aërial Organs of the Skin.—Special organs developed from the outer surface of the body and adapted to air-breathing are confined to
| Fig. 14.—Spiracle of Cockchafer-Grub. | Fig. 15.—Spiracle or Fly. |
the articulates and mollusks. None are found in the radiates, this great division being wholly aquatic.
Although the crustaceans always possess gills, and are theoretically classed as water-breathers, yet certain species live entirely on land, and some are even killed by long immersion in water. The gill-chambers of the land-crabs are proportionately very large, exposing more surface to the air, and the openings are small to prevent evaporation. 
Fig. 16.—Spiracle Leather-Coat. Some species inhabit the highest ground of West India islands, but they seek damp or sheltered places, and possess some means of keeping the gills moist, either by carrying a small quantity of water, or by a secretion of the sponge-lining of the chamber. Structurally these organs are gills, but functionally they are true lungs.
Another crustacean, the wood-louse (Oniscus), lives in damp places, and breathes air by foliaceous gills beneath the abdomen.
The only groups of articulates wholly air-breathing are the myriapods, insects, and arachnids. The myriapods show a transition from the skin-respiration of the leech and earth-worm to the air-tubes of the insect. The latter possesses the most peculiar and effective system for aeration. As in
