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 Beachiopoda. 



The function of absorption in the Polyzoa and the BracMo- ' 

 poda is not so highly differentiated as the Myriapoda, Imccta, 

 Arachnida, 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 

 Brcwliiopoda, the function of absorption is analogous to that of 

 the Actinozoa. There is an alimentary ca,nal 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. 



'J'he Mollusca. 



The function of absorption in the Mollusca is placed under 

 the liead 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 Imrrtcbmta. 

 But although there is no special apparatus set apart for 

 absorption, the nutrient fluids, absorbed by either the sarcode, 

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

 required, the distribution being in some animals eifected 

 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 remore 

 •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. 



