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 circulate.s 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(3.5S<2^1. .- 



afferent branchial 

 arter/. 



branchio^tcgite 



from, lateral 

 bloocC $inix.$ 



l^jrtion of gills of crayfish. 



protecting branchiostegite. 

 blood aerated .3 



Note the 

 How is the 



