Chap, vi.] LUNGS OF CRUSTACEA. 229 



air-breathing forms closely allied to those that breathe 

 oxygen dissolved in water ; not only are there true 

 amphipnous Vertebrates (see page 236), but there are 

 among the Crustacea some terrestrial Isopods, in 

 which some of the appendages are placed in a cavity 

 of the abdomen, which is partly closed ; the cavity of 

 such of these appendages as are not rudimentary 

 opens to the atmosphere by a longitudinal slit. Among 

 the true crabs there are also some forms that con- 

 stantly live on land, such as the robber crab (Birgus 

 latro), the land crab (Gecarcinus), and others ; the 

 gills in the branchial chamber of these Crustacea are 

 always small, but a quantity of air is to be found in. 

 the chamber ; in Birgus this chamber is divided into 

 a lower and smaller one, which contains small 

 gills, and an upper larger one, which never con- 

 tains any water, but always air, and which has 

 its walls not only richly supplied with blood-vessels, 

 but also produced into branched outgrowths, or villi, 

 in which the biood- vessels are particularly well de- 

 veloped. 



It is not in the lower Metazoa alone that the 

 lining of the alimentary canal serves as a means of 

 entrance for oxygen ; even among the Vertebrata, 

 where, in the higher forms, the respiratory organs 

 (lungs) are really outgrowths of the enteric tract, we 

 know of a loach (Cobitis) which swallows air bubbles ; 

 among the Echinoderms, the surface of the mucous 

 membrane of part of the intestine is, in Stichopus 

 variegatus, so grooved as to display a large amount of 

 vascular surface to the action of the inflowing water, 



O 



or, as in many (Holothuria, Cucumaria), special 

 branched organs, which extend throughout the greater 

 part of the length of the body, are developed from the 

 walls of the cloaca ; in such pneumonophorous holo- 

 thurians water is pumped in and out by the muscles 

 at the hinder end of the body. Such a form of anal 



