230 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



respiration is not confined to the Echinodermata, for 

 it is very common among the Arthropoda ; among the 

 Entomostraca it has been frequently observed; in 

 Leptodora, when the intestine is free of food, the 

 water appears to pass in a continuous stream into the 

 more anterior parts of the tract, and, when the 

 stomach is full, water is taken in and expelled by the 

 mouth ; anal respiration has been seen by Lereboullet 

 in the crayfish, and the rhythmical closing and 

 dilatation of the rectum may always be seen in that 

 animal, after the extirpation of the thoracic ganglia. 



Among Worms, enteric respiration obtains in the 

 Rotatoria, in some, if not all, Gephyrea, and in some 

 of the aquatic Oligochseta ; Dentalium is the only 

 mollusc in which it has been definitely observed. 



In certain polychsetous Annelids, a somewhat 

 complex arrangement has been detected by Eisig, 

 which is of especial interest, both morphologically and 

 physiologically, when it is compared with the function 

 and structure of the respiratory organs of Vertebrates. 

 The observation that a number of air bubbles constantly 

 escape from the mouth or anus of Hesione sicula led 

 him to detect the presence of distinct outgrowths of 

 the intestine, which clearly serve as air reservoirs, and 

 at the same time may be used as floats or hydrostatic 

 supports ; they are especially of use when the intestine 

 is filled with food, and structural evidence in favour 

 of their respiratory significance is afforded by their 

 rich supply of blood-vessels. In connection with this 

 it is very interesting to note that a fish will use up all 

 the air in its air bladder before it dies of suffocation, 

 and that, conversely, in the pulmonate Yertebrata the 

 lungs have undoubtedly the power of acting as 

 hydrostatic supports for the body, when immersed in 

 water. 



The Cliordata present us with the most interest- 

 ing and instructive series of arrangements, for while 



