86 SPONGES 



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 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 forms, such as 

 Ascons, microscopic food particles are ingested by the collar cells 

 which line the whole gastral cavity ; 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, to use Carter's expressive phrase. 



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 finally be 

 usurped almost entirely by cells in the parenchyma ; the collar cells 

 then become concerned only with the production of the current, 

 their ingestive activities being in abeyance (MetschnikofT). 



It should be added that, according to the investigations of 

 Loisel [10], some sponges, at least, are able to absorb nutriment 

 in solution, as well as in suspension. The cells of the epithelium 

 exercise in such cases a selective power, well shown by experiments 

 with stains acting intra vitam ; some substances are permitted to 

 pass through the epithelium into the parenchyma, while others are 

 excluded. 



Digestion is in most cases intracellular, ingested bodies being 

 absorbed within cell vacuoles, as in Protozoa. It is possible, how- 

 ever, that, in the case of bodies too large to be so ingested, a kind 

 of intercellular digestion takes place. Lieberkuhn, whose accuracy 

 as an investigator is above suspicion, saw Infusoria surrounded by 

 wandering cells in the canals of Spongitta, and there gradually 

 absorbed. 



Circulation and distribution of nutriment is effected partly by 

 wandering cells, partly, there can be no doubt, by the mesogloea, 

 which acts as an internal medium between the cells and tissues. 

 Loisel compares the mesogloea from the physiological point of view 

 to the interstitial lymph of higher animals. Substances, either 

 solid or fluid, are cast out into it from the cells, and then taken up 

 again by other cells. On the other hand, the transport, especially 



