436 LIFE : OUTLINES OF GENERAL BIOLOGY 



interesting process in itself, since it is due to the contraction of 

 spindle-shaj^ed smooth muscle-cells which form a sphincter round 

 the orifice. As there are no specialised nerve-cells, these contractile 

 cells must be neuro-muscular, i.e. "receptors" as well as "effectors". 

 But if a microbe or an irritant particle finds entrance to the interior 

 of a sponge, it is captured by the flagellate cells lining the canal 

 system ; and it may be thence passed on to mobile phagocytic cells 

 in the mesogla^a. By feeding sponges with recognisable material, 

 such as carmine particles or milk, then killing and sectioning them 

 at different intervals of time, it has been possible to determine 

 various stages in the capture and transport — from canal cells to 

 the pliagocytes, and from one part of the median sponge-tissue 

 (mesogl(i.'a) to another. 



In Hydra, where there is practically no mesogloea, but simply 

 a two-layered arrangement of ectoderm and endoderm, the flagellate 

 and ama'boid cells lining the food-cavity act not only as food- 

 capturers, but as "stationary phagocytes". More definitely than in 

 Sponges, the two functions of intracellular digestion and phago- 

 cytosis are combined. 



In higher Ccclenterates the food-capturing and digesting functions 

 are discharged, as in Hydra, by the endoderm cells lining the food- 

 canal, but most of them have what sponges have, and Hydra has 

 not, wandering amoeboid cells in the mesogloea, which deal with 

 intruded microbes, small parasites, and irritant particles. 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 lining the body- 

 cavity or ccclom, or else they float in the perivisceral fluid, perhaps 

 also in the blood. They may have several functions, respiratory and 

 e.xcretory for instance, but the phagocytic function is of great 

 importance all the greater because the cells lining the food-canal 

 have now lost the Hydra's power of intra- cellular digestion. 

 That is to say, they do not engulf solid particles as such, to be 

 digested in their interior; they are only able to absorb the fluid 

 into which the food has been changed by the action of the digestive 

 ferments. 



Crustaceans, insects, molluscs and the like have a more or less 

 well-develojx'd blood- vascular system, and there are often colourless 

 amoeboid cells in the blood like the leucocytes of most Vertebrates. 

 But the phagocytic function still depends, largely at least, on 

 wandering amfcboid cells in the body-cavity fluid or in the meso- 

 dermic tissues. Yet again, as the vascular system in Arthropods and 

 Molluscs is usually in part lacunar, that is in ill-deftned spaces, 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 of an Invertebrate exhibit the power of migrating 



