HOW ANIMALS DIGEST. 



605 



of all the other vegetative functions. A single opening serves both 

 to receive the food and expel the waste matter. Within even this 

 narrow limit of structure, we find a host of low animals which exhibit 

 a great variety of forms ; so that from the hydra to the ctenophore is 

 a progressive series showing a gradual specialization of this common 



Mould, or Proto- 

 coccus-Cell. 



Kernel. 



Fig. 4. 

 Pond-Amceba digesting its Food. 



Amceba eating. 



organ. In the higher part of the series, as for example the sea-anem- 

 one, thei'e is a digestive cavity somewhat separated from the body 

 cavity, though still connecting ; and all the excretions have to find 

 their way out through the oral aperture. 



In the compound hydrozoans, produced by budding and division, 

 such as sertularia and the so-called corals, the body cavity is continu- 

 ous through the whole community. Hence each individual (though 



Pig. 5. Perpendicular Section op Actinia holsatica (after Frey and Leuckart) : a, month ; b, 

 gastric cavity; c, common cavity, into which the gastric cavity and the intermesenteric cham- 

 bers open; d, intermesenteric chambers; e, thickened free margin, containing thread-cells of, 

 /, a mesentery ; g, reproductive organ ; h, tentacle. 



it is scarcely correct to regard it as such) has its stomach connected 

 with the stomachs of all the others. Whatever food one digests serves 

 to nourish the whole colony. They are absolute communists. 



At this point should be presented the fact that in all animals the 

 lining or secreting membrane of the food-canal is essentially but a con- 

 tinuation of the skin. That such is true of the amoeba is evident, for 

 what was the outside of the body-mass becomes when food is enveloped 

 the lining of the new cavity. The cup-shaped body of the hydra can 



