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 blood-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, 

 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 



