212 



Comparative Animal Physiology 



because it will occur by necessity through all permeable membranes when 

 the partial pressure of the gas establishes a sufficient gradient. 



Special respiratory mechanisms have evolved in some annelids (Nereis and 

 Chaetopterns), but many members of this phylum respire through the general 

 body surface. In the oligochaete, Drilocritis, a specialized "dorsal groove" in 

 the epithelium permits the drawing of air bubbles below the water surface and 

 the absorption of oxygen through this portion of the skin."'' The pulmonale 

 gastropods (Limnaea and Helicosovia) can depend on their integuments when 

 submerged for long periods of time, particularly at lower temperatures, al- 

 though usually they take in aerial oxygen through the lungs.*"'" Fraenkel and 

 Herford^^^ showed in blowfly larvae (Calliphora) that under experimental 

 conditions 10 per cent of the total oxygen intake could be accounted for by 

 cutaneous channels. Embryos, young larvae, and some "transparent" arthro- 

 pods also respire through the body surface. The fish Acara (Chromidae) under- 

 goes caudal differentiation with great increase in vascularity so that the tail 

 becomes a specialized respiratory structure. 



From the time of Spallanzani, numerous investigators""' -^°' -^^ have studied 

 the problem of gas exchange through the integument of vertebrates, many of 

 which respire through the skin, particularly in eliminating carbon dioxide 

 (Table 35). According to Krogh the eel's skin can provide 60 per cent as much 



TABLE 35. CUTANEOUS RESPIRATION OF SEVERAL VERTEBRATES''' 



(Respiratory Exchange expressed as cc./sq. dcm./hour) 



TABLE 36. COMPARISON OF CUTANEOUS AND PULMONARY 

 RESPIRATION IN FROGS AT 20° C. [AFTER KROGH''") 



(Respiratory exchange expressed as cc./kg./hr. *) 



oxygen as is provided when the gills and skin both function in normal respini- 

 tion. This is sufficient to support the fish for an indefinite period at low 

 temperatures but is inadequate above 15° C.-" In man and pigeons less than 

 1 per cent of the oxygen intake is through the integument. 



Studies have demonstrated species differences in the proportion oi cutaneous 

 to pulmonary respiration in frogs (Table 36) and have disclosed the relative 

 * Morrison'''' has utilized the less ambiguous mathematical expression, cc./kg.hr. 



