26 



RESPIRATORY MECHANISMS 



suitable. The flagellates show a preference for low oxygen 

 tensions, but avoid water from which oxygen has been com- 

 pletely removed. The luminous bacteria require only a very 

 low concentration of O2 to emit light. The experiment by 

 Thorpe illustrated in Fig. 6 shows that the first instar larvae of 

 the ichneumonid Omorgus absorb O2 only through the body 

 proper and especially through the fore part, but not into the 

 "tail." 



c. 



Fig. 6. First instar larva of Omorgus in Polytoma culture. 

 B, after 7 min.; C, after 10 min.; and D, after 15 min. 



A, after 6 min. 

 (Thorpe.) 



In a very large number of forms belonging to all major 

 groups some oxygen is absorbed through the skin and C0 2 is 

 eliminated, generally in even larger amount. When there is 

 a tension difference, as is practically always the case, and when 

 the integument is permeable, some gas exchange is unavoid- 

 able and all gradations are found between, this gas exchange 

 being respectively essential or negligible in the economy of 

 the animal. The accessory cutaneous respiration in certain 

 animals will be discussed below (p. 54). 



Specific respiratory organs show a specially enlarged surface 

 in contact with the external medium through which more 



