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collar cells, another set rcgarding the „mesodenn cells" as the 

 true phagocytes, Those wlio hold the formcr view cxplain the 

 presence of ingested particles in mosoderm cells as having passed 

 on to tliem by the collar cells. The true explanation seems to 

 lie, as Metschnikopf has pointed out, between these two opinions, 

 The „mesoderm" shows a great difference as regards its degree 

 of evolution in different types. While in some, eg. Ascons, tlie 

 parenchyma is scarcely developed, in ethers it reaches a high 

 grade of complication. In accordance with these differences the 

 part played by the parenchyma in capturing food may, in some 

 cases, be very slight, in others very great. There can be no 

 doubt whatever, from numerous experiments that have been per- 

 formed by various investigators from Carter and Lieberkühn 

 in the fifties up to Yosmaer and Pekelharing at the present 

 time, that in many sponges at least the collar cells are very 

 active in capturing food. On the other hand, these cells are from 

 their nature and size incapable of ingesting large bodies such as 

 Infusoria or Diatoms, Food of the latter kind could only be 

 absorbed by becoming entangled in the webs of tissue in the 

 incurrent canal system, there to be absorbed by the phagocytic 

 wandering cells, or, it may be, by porocytes". 



„Considered generally, sponges present a gradual evolution as 

 regards the power of ingesting food materials, corresponding to 

 the evolution of the canal system. In the simplest ferms, such 

 as Ascons, microscopic food particles are ingested by the collar 

 cells ; larger bodies, such as diatoms may be captured by the 

 porocytes, which close upon them like a trap when they enter 

 the intracellular lumen of the pore. The collar cells represent 

 however the chief „eating organ" of the sponge". 



„In other sponges the complications of the incurrent system 

 represent a progressive elaboration and perfection of an apparatus 

 for assimilation, doubtless, in the first instance, of bodies too 

 large to be absorbed by the collar cells. As the water passes 

 through the inhalant canals and spaces, food in it is captured 

 by cells in the parenchyma, either by phagocytic amoebocytes 

 or, perhaps, also by porocytes. The function of ingestion may 



