METAZOA 139 



iv. Certain organs are formed in different animals from different 

 layers. Organs of respiration may be covered or lined by ectoderm, 

 as are the gills of crustaceans and annelids, the external gills of the 

 tadpole, and the lungs of snails ; or by endoderm, as are the gills of 

 a fish or the lungs of vertebrates. The skin, when it is naked or 

 covered only by thin cuticle is always respiratory, and in many small 

 animals is the only organ of respiration. Cilia may keep water in 

 movement over it so as to renew the supply of oxygen, and when 

 there is a vascular system a rich blood-plexus may increase the 

 efficiency of the skin, as in earthworms. In larger animals there are 

 usually localized organs of respiration. In these the surface is in- 

 creased by folding or branching either outwards or inwards, the blood 

 supply is richer than elsewhere, and there is some means of con- 

 stantly renewing the medium. Organs of aquatic respiration are 

 usually projections, known as gills : the water around them is renewed, 

 either by muscular movements of the body, of limbs which bear the 

 gills or of structures in their neighbourhood, or by ciliary action. 

 Organs of aerial respiration may be those that were originally used 

 for aquatic respiration: this is especially the case when they are en- 

 closed in a chamber which protects them; sometimes, as in snails 

 and land crabs, such a chamber becomes itself converted into a re- 

 spiratory organ by the vascularizing of its lining. In other cases, as 

 in terrestrial vertebrates, there are developed for this function cavities 

 in the body into which air is drawn. In order to prevent damage to 

 the epithelium by desiccation, a layer of moisture is always main- 

 tained over air-breathing surfaces, either by exudation or by special 

 glands or by water-retaining hairs, etc. Consequently aerial respira- 

 tion is in the long run aquatic respiration, with the difference that the 

 supply of oxygen in the layer of water over the respiratory epithelium 

 is maintained not by renewal of the layer but by diffusion from ad- 

 jacent air. It is maintained that on that account aerial respiration is 

 less efficient than aquatic, and this argument is supported by the fact 

 that the respiratory area of air-breathing animals is greater than that 

 of related forms which have aquatic respiration. 



What has been said in the foregoing paragraph does not apply to 

 the tracheate arthropods, whose respiration does not take place 

 through an epithelium with the intermediation of the blood, but 

 by the bringing of air directly to the tissues by a system of fine 

 ectodermal tubes — the tracheal system (p. 440). 



The relative effect upon respiration of the pressures of carbon 

 dioxide and of oxygen differs according as the animal is living in water 

 or in air. In clean waters the pressure of carbon dioxide varies little, 

 because any excess of the gas is removed by the formation of carbon- 

 ates and bicarbonates, but the pressure of oxygen is easily lowered as 



