PHYSIOLOGY OF THE INVERTEBRATA. 209 



liquids, and during contraction send out radiating canals. 

 This system probably communicates with the exterior. By 

 this primitive respiratory organ the working tissues are 

 brought into contact with oxygen dissolved in water. 



The Poeifeea. 



In the Porifera (Spongida) respiration is effected by means 

 of the oxygen dissolved in the water, which permeates 

 through the various canals, and thereby brings it into 

 intimate relation with the whole mass. In the circulation of 

 this water through the ordinary fresh-water sponge (Spongilla) 

 there is a fusion of the functions of digestion, circulation, 

 and respiration. " Sponges absorb oxygen and give off 

 carbonic anhydride with great rapidity ; and the manner in 

 which they render the water in which they live impure, and 

 injurious to other organisms, suggests the elimination of 

 nitrogenous waste matter." It is possible that the oxygen is 

 retained in the substance of a sponge by certain respiratory 

 pigments — probably a histohaBmatin. Sponges are rich in 

 chlorophyll, but this pigment has another function — viz., the 

 formation of fatty matter.* 



The Ocelenteeata. 



In the lower Godenterata the function of respiration is 

 performed by the general surface of the body. The fluids in 

 these animals are in close relationship to the water in which 

 they live ; and consequently the ectodermic lining serves as an 

 organ of respiration. In other words the lower Ocelenterates 

 respire by the skin. In some of the higher orders of this 

 group the respiratory function is performed in the water- 

 vascular tubes along with other functions performed by the 

 same vessels. 



But there is no doubt that the chief mode of respiration in 



* MacMunn in Journal of Phy Biology, vol. 9. 



