PHYSIOLOGY OF THE INVERTEBRATA. 121 



more complicated than in the RMzopoda. The digestive 

 and somatic cavities are not differentiated, for they form one 

 common cavity. The digested food is absorbed by the cells 

 of the endoderm. In the Frotozoa the function of absorption 

 is effected by the sarcode, whereas in the Hydrozoa the 

 " Barcode " becomes differentiated into cellular membranes, 

 the internal one (endoderm), lining the digestive cavity. 

 The endoderm of the Hydrozoa is the absorptive layer 

 and is the means of transferring the absorbed fluids to the 

 ectoderm. 



Although there are many points in the mode of absorption 

 in the Hydrozoa comparable to those of the RMzo-poda, 

 yet the former class marks a distinct advance on that of the 

 latter ; for the food is first digested in the " chylaqueous " 

 fluid contained in the digestive and somatic cavity, whereas 

 in the HMzopoda the food particles are brought into actual 

 contact with the sarcode, which performs both the functions 

 of digestion and absorption. 



In the Adinozoa the function of absorption comes under 

 the second method already described. The digested food 

 transudes through the linings of the digestive cavity, or 

 passes directly through the posterior aperture into the somatic 

 cavity. The somatic cavity, which is distinct from the 

 digestive cavity,* contains a " chylaqueous " fluid. This fluid 

 consists largely of water, and contains albuminoid spherules, 

 which are possibly the precursors of the white corpuscles of 

 chyle and of blood in the higher animals. 



The nutrient matter of the digested food, having passed 

 into the somatic cavity, is absorbed by the endodermic cells 

 of that cavity and by the mesenteries* 



The Echinodermata. 



As far as the function of absorption in the Echinodermata 

 is concerned, there is very little difference from that of the 



* In Actinozoa the digestive cavity is suspended in the somatic cavity. 



