BESPIRATIOX OF THE COPEPODA. 85 



plius, it is easy to make out the origin of the arrangement 

 in the latter. The ciliated funnels and lateral canals of 

 the former can only be of service when there is a thin 

 unchitinized anterior surface through which water can 

 transude into the coelom : by the extension of chitiniza- 

 tion over the whole surface these organs lose their func- 

 tion and abort, while the cloacal '^contractile vesicle" 

 takes on an inspiratory as well as an expiratory function, 

 and becomes more or less confounded with the rectum, 

 from which probably, even in Rotifers, it takes origin. 



Here must be noticed the wide diffusion of anal respira- 

 tion in aquatic insect larvae (alternate inspiration and 

 expiration by the pumping movements of the rectum). 

 This would point to a common origin with Crustacea. 



A list of the groups in which anal respiration is made 

 out may be added. 



Vermes : 



Rotifera. 

 Gephyrea. 

 Oligochseta Limicola. 



ECHIXODERMATA : 



Holothuroidea. 



Arthropoda : 



Crustacea (general). 

 Insecta (most aquatic larvae). 



