Respiration and Metabolism 215 



Specialized epithelial modifications are not uncommon in supplementing 

 the activity of the gills. The hermit crab, Coenohita, has such a region on the 

 ventral side of the anterior part of the body which must carry on much of the 

 respiratory exchange in the absence of a well developed gill system. Long ago 

 Birgus Ultra, a terrestrial robber crab, was shown to possess a modified gill 

 chamber in which vascularized epithelial appendages on the roof of the cavity 

 supplement respiratory exchange by the reduced gills. 



The woodlice, Isopoda, represent a seriation from aquatic to terrestrial 

 existence, and the gill-like structures on the endopodites of the pleopods show 

 increasing adaptations from aquatic to aerial respiratory structures in, for 

 example, the three forms: Ligia, which is essentially amphibious and hygric; 

 Oniscus, which dwells in damp places; and the more xeric Porcellio, which 

 prefers drier conditions."^'' •^*'* The endopodites of aquatic isopods have thinner 

 walls than do those of terrestrial species. The latter may develop systems of 

 capillary grooves which permit moisture to drain either toward the rectum for 

 absorption or toward the pleopods to moisten the respiratory structures. 



In Aselhis and Porcellio the "white body" or "tracheal organ" functions as a 

 respiratory structure particularly concerned with the elimination of carbon 

 dioxide. ^''^ Extirpation of these structures from the pleopods causes death 

 within a few hours to several days. 



Certain groups of insects possess tracheal gills, either aquatic or aerial, in 

 which a tracheal system replaces the blood vascular system (Ephemeridae, 

 Odonata, Trichoptera, Coleoptera). Experimental procedures applied to may- 

 fly nymphs indicate that the caudal gills play an important part in the respira- 

 tory exchange.-^"' •"- If the gills are removed from Hexagenia reciirvata, the 

 oxygen uptake is decreased 50 per cent. The damsel fly naiad (Enallagma) 

 has been shown by extirpation experiments to take up oxygen normally 

 through the caudal gills and secondarily through the body surface.-^** In 

 normal animals oxygen is removed from the medium down to 2,4 per cent 

 saturation, whereas in gill-less individuals oxygen is removed from the medium 

 only down to 14.5 per cent saturation. In the caddis fly, Macronema, on the 

 other hand, gill removal results in little or no reduction in oxygen consump- 

 tion.-^^ 



The pharyngeal gill structure of prochordates is elaborate, but efficiency 

 seems to have yielded to size. Amphioxiis has well over 100 functional ciliated 

 branchial bars. The multi-slotted branchial sac of the tunicate is also well 

 equipped with cilia to provide for the circulation of water through the gill slits 

 and over the vascularized gill filaments. 



In the vertebrates the gills are either external, as in embryonic lungfish and 

 Amphibia, or enclosed within a chamber as in the teleosts. The adult lamprey 

 with its peculiar jawless sucking mouth has covered gills, and these are ven- 

 tilated by water which passes both in and out of the several gill openings. The 

 male lungfish, Lepidosiren, develops temporary pelvic gills during the breeding 

 season when he assumes custodianship of the eggs and young and remains 

 below surface in oxygen-deficient water.- It has been claimed that the oxygen 

 content increases in the water around the eggs, a result of actual oxygen 

 secretion by the gill filaments.^-- ^'^ Gills reach their greatest development 

 among the teleosts and, considering the respiratory area, have been regarded 

 as surpassing the lungs in efficiency.*'" Gill efficiency, however, depends on 



