314 



THE MAINTENANCE OF THE INDIVIDUAL 



inte-stlne/ 



respiratory 



body vail 



are found in many spiders while a similar structure called a gill- 

 book occurs in the horseshoe crab, Limulus. Gill-books may more 



properly be considered as 

 "^'^'^ modified gills. 



The Body Surface. 

 This type of respiratory 

 system is probably the 

 most simple. It consists 

 of an exchange of gases 

 through the surface of 

 the body. It is found, 

 however, not only in such 

 simple one-celled animals 

 as the protozoa, which 

 have no specialized sys- 

 tem for respiration, but 

 also in sponges and coe- 

 lenterates. Even in the 

 parasitic and free-living, 

 flat worms and some 

 roundworms, respiration 

 is of the same type. 

 Some of the smaller forms 

 of the higher groups may 

 also resort to this method 

 of gaseous exchange. 



In some of the more 

 highly specialized forms 

 such as the earthworms, a circulatory system is present although 

 respiration still takes place through the cuticle. The blood of the 

 earthworm is red and contains hemoglobin which is dissolved in the 

 plasma, just the opposite of the situation in the vertebrates where 

 hemoglobin occurs in the red blood cells. 



Complete dependence upon integumentary respiration does not 

 occur among vertebrates. Probably the closest approach to such a 

 situation is in the lungless salamanders (Plethodontidae) and in 

 certain other urodeles, such as the hellbender, Cryptohranchus. In 

 the former, integumentary respiration is usually supplemented by a 

 capillary network in the pharyngeal region and is therefore designated 

 as buccopharyngeal respiration. A highly developed system of capil- 



Cl. 



The 



OCXCCL 



'lungs," or respiratory tree of the sea 

 cucumber, a holothurian. 



