750 COMPARATIVE PHYSIOLOGY. 



In Sponges, infection is often avoided and parasites are 

 excluded by the closure of the inhalent pores. But if 

 entrance be effected, the microbe or irritant is dealt with by 

 the amoeboid cells of the middle stratum, which have also 

 to do with ordinary digestion. Thus disease in Sponges is 

 very rare. In Hydra, where there is virtually no mesoglcea, 

 the flagellate or amoeboid cells lining the gut act as so many 

 "stationary phagocytes." Thus, in these two cases, the 

 functions of intra-cellular digestion and of " phagocytosis " 

 are combined. 



In other Coelentera, as in Hydra, the ordinary digestive 

 functions are restricted to the endoderm cells lining the gut, 

 but most of them have, what Hydra has not, wandering 

 amoeboid cells in the mesoglcea, and these deal with 

 microbes, parasites, and irritants. The same is true of 

 simple worms, such as Turbellarians. 



In higher w r orms and in Echinoderms the phagocytic cells 

 are usually situated on the peritoneal epithelium, or float in 

 the perivisceral fluid. They may have many functions, 

 respiratory and excretory, for instance, but the phagocytic 

 function is of great importance, all the more so that the 

 gut has now lost its power of intra-cellular digestion. 



Crustaceans, insects, molluscs have a more or less well- 

 developed blood vascular system, and there are often 

 amoeboid cells in the blood like the white blood corpuscles 

 of most Vertebrates. But the phagocytic function still 

 depends, largely at least, on wandering phagocytes in the 

 body cavity or in the mesodermic tissues. But as the 

 vascular system in these forms is usually lacunar, no rigid 

 distinction can be drawn between phagocytes in the blood 

 and phagocytes in the body cavity. No case is known, 

 however, in which the leucocytes or white blood corpuscles 

 of an Invertebrate exhibit the power of migrating through 

 the walls of the blood vessels to the seat of irritation or in- 

 jury, as is common among Vertebrates. 



Among Vertebrates, as the circulatory system becomes 

 gradually more highly developed from Tunicates onwards, 

 the number of extra-vascular phagocytes is reduced, and 

 more and more depends on those of the blood. In the fin 

 of a young newt an injury or an infection may be dealt with 

 solely by the migratory phagocytes of the connective tissue ; 



