. Biology; the story of living things. THE METABOLIC MACHINERY OF ANIMALS 315 giU. laries which almost penetrate to the outer surface of the epidermis is found in the integument of many amphibians. In some amphibia as much as 74 per cent of the carbon dioxide is given off through the skin. Such adaptations are possible only where a cool environment keeps down the metabolic rate of these forms. Gills. Gills are either flattened or feathery, and are external or internal in their location. Invariably the blood in them and is separated from the surrounding water by a thin membranous wa
. Biology; the story of living things. THE METABOLIC MACHINERY OF ANIMALS 315 giU. laries which almost penetrate to the outer surface of the epidermis is found in the integument of many amphibians. In some amphibia as much as 74 per cent of the carbon dioxide is given off through the skin. Such adaptations are possible only where a cool environment keeps down the metabolic rate of these forms. Gills. Gills are either flattened or feathery, and are external or internal in their location. Invariably the blood in them and is separated from the surrounding water by a thin membranous wall through which the dissolved gases are exchanged. Among the invertebrates the position of the gills varies in accordance with the habitat of the animal. In such forms as the crayfish for to pericardial example, they are in a protected Sirjus outer chamber covered by chitin. Circulation is accomplished by the creation of a water current through the action of the swim- merets and certain appendages about the mouth. In fishes and amphibians, water typically enters the mouth where it is passed to and o^^er the pharyn- geal gills and from there through slits to the outside. Tracheae. These are com- posed fundamentally of air- carrying tubules, which, by a series of anastomoses and rami- fications, penetrate to nearly all parts of the body. They are characteristically found in most insects, myriapods, protracheates, and some arachnids. Such a sys- tem starts with a series of openings known as spiracles, occurring along the outer surface of the thoracic and abdominal segments. Leading from the spiracles are air tubes, or tracheae, which show great numbers of anastomoses, frequently forming abdominal reser- voirs, or air sacs. The tracheae are nothing more or less than a series of pipes, for they are lined with chitin and stiffened by a spiral, fiberlike thickening:. The finer subdivisions of the tracheae extend. efferent branchial V(;2^1. .- afferent branchial arter/. branchi
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