SPONGES 87 



of solid materials, is effected largely by the wandering cells, which 

 are capable of active migration. 



Excretion in sponges is still a disputed point. Bidder ascribes it to the 

 porocytes. Other authors attribute this function to the choanocytes, especi- 

 ally in those forms in which the parenchyma is most active in the capture of 

 food. Loisel regards the mesogloea as performing the function of excretion 

 by its own activity. Yacuoles and lacunae containing matter to be excreted 

 arise in it and are emptied to the exterior by contractions of the mesogloea 

 itself, aided by cell contractions. The matter must at present be considered 

 very doubtful. There can, however, be little doubt that the wandering 

 cells play a considerable part in excretion as well as in other functions. 



Animal Functions. Sponges in correspondence with the absence 

 of a special nervous system show a great lack of co-ordination in the 

 activities and movements of their cells. Thus the flagella of the 

 collar cells do not beat in unison like the cilia of the epithelia in higher 

 animals, but each works independently of the others (Vosmaer and 

 Pekelharing [30]). 



Sensitiveness to external conditions is often exhibited in a marked 

 degree, but in such cases each cell placed superficially possesses this 

 quality equally, and there is no class of cells marked out as sense 

 cells by the possession of special physiological or structural characters. 

 Contractility is probably a quality possessed by all sponges to a 

 certain extent, and in some it is greatly developed. In all cases it 

 appears to reside in the cells of the epithelial stratum of the dermal 

 layer. Bidder, however, regards the power of contraction as 

 largely due to elastic tension of the mesogloea, tending to bring about 

 a contraction of the sponge if not opposed by the activity of the 

 canal system. This, however, would hardly explain the epithelial 

 sphincters often present. 



Loisel, as we have seen, considers the mesogloea not merely endowed 

 with passive elasticity, but as actively contractile. This would necessitate 

 a very different view of the nature of the ground substance from that 

 generally held, and requires confirmation before it can be accepted. 



Statements have sometimes been made to the effect that the current of 

 the canal system may be reversed and flow into, instead of out from the 

 osculum. If these statements are not simply due, as is very probable, to 

 erroneous observations, they might perhaps be explained, as Vosmaer and 

 Pekelharing suggest, as follows. If, in a sponge with several oscula, one 

 of them is pouring out a very strong current, it might act as a flue, so to 

 speak, and cause the current in the other chimneys (oscula) to stop or even 

 to flow inwards. The authors mentioned have also put forward a theory 

 of the cause of the current through the canal system different from that 

 generally adopted. According to their view the action of the flagella alone 

 is incapable of causing a definite and continuous current, since they are 

 not co-ordinated. The current which can be observed flowing out of the 

 osculum is brought about by the disposition of the pores and the oscular 



