124 PHYSIOLOGY OF THE INVERTEBRATA. 



vessels are known to convey the digested food into the circu- 

 latory system "than the irregular venous receptacles which 

 are in contact with the parietes of the intestine." 



The Polyzoa and BuACHioroDA. 



The function of absorption in the Fohfzoa and the Brachio- 

 poda is not so highly differentiated as the Mjiriajpoda, Inscda, 

 Araehnida, and Crustacea. In the latter, the digested food 

 passes into vessels, or, in other words, into the circulatory 

 system ; but as there are no vessels in the Polyzoa and the 

 Brac]iiopod((, the function of absorption is analogous to that of 

 the Acthwzoa. There is an alimentary caiial suspended in a 

 somatic or body cavity. The digested food transudes through 

 the walls of the digestive system, and is then absorbed by the 

 external endoderm of the body cavity, as well as by the organs 

 suspended therein. 



The Mollusc A. 



The function of absorption in the MoUusca is placed under 

 the head of our third category. The digested matter is 

 absorbed by vessels, but these perform the dual functions of 

 absorption and circulation. 



There are no special absorbent vessels in the Inrcrtebrata. 

 But although there is no special apparatus set apart for 

 absorption, the 2iutrient fluids, absorbed by either the sarcode, 

 somatic linings, or blood-vessels, are spread wherever they are 

 required, the distribution being in some animals effected 

 slowly, in a way analogous to absorption. In others the 

 distribution of the nutrient fluids is accomplished rapidly 

 by the establishment of currents, which serve also to remove 

 the excretory products eliminated from the organs. This 

 originates another function, the circulation of the blood, and 

 another set of organs by which this is performed. 



