754 COMPARATIVE PHYSIOLOGY. 



of. This is particularly true of the amoeboid Protozoa, the 

 Rhizopods. Sometimes, moreover, the bacteria or other 

 micro-organisms which produce disease are actually avoided, 

 for some of the Protozoa exhibit that sensitiveness (or 

 chemotaxis) which distinguishes the wandering amoeboid 

 cells or phagocytes of higher animals. Thus a Myxomycete 

 will creep towards a decoction of dead leaves and away 

 from a salt solution, and will " prefer " a nutritive fluid 

 which is not swarming with bacteria to one that is. 



In Sponges, infection is often avoided and parasites are 

 excluded by the closure of the inhalant 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 intracellular digestion and of "phagocytosis" 

 are combined. 



In other Ccelentera, 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 worms 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 intracellular 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 



