ORIGIN OF THE NERVOUS SYSTEM 59 



we may turn first to those very primitive metazoans, the sponges. The 

 body of one of the simpler sponges is a more or less goblet-shaped, 

 multicellular mass, whose surface is covered with an enormous number 

 of minute pores ; these lead into tubes which in turn communicate with 

 a relatively large central cavity that opens to the exterior by an aper- 

 ture of considerable size, the osculum. In a living undisturbed sponge, 

 water is continually passing into the lateral pores, through the tubes 

 and central cavity, and out at the osculum. This current is produced 

 by means of numerous cells, the choanocytes, which are provided with 

 vibratile lashes and are variously distributed through the iriternal 

 chambers and tubes of the sponge. Apparently these choanocytes work 

 incessantly, and the current generated by them carries food, etc., to 

 the sponge and removes waste products. Although frequent efforts 

 have been made to show that nervous structures occur in sponges, 

 nothing of this nature has been conclusively demonstrated and it is now 

 generally believed that these animals are without differentiated nervous 

 organs, either sensory or central. Nevertheless, sponges are capable of 

 a certain amount of response. Merejkowsky (1878) observed that when 

 he pricked with a needle the inner face of the osculum of Rinalda, this 

 aperture quickly closed, not to open again for several minutes. The 

 same reaction occurs with the lateral pores of many sponges (Vosmaer 

 and Pekelharing, 1898). This power of closing the pores seems to be 

 the only means by which a sponge may check the current which ordi- 

 narily flows through its canals, for, as already mentioned, the choano- 

 cytes apparently lash the water incessantly. 



When a search is made for the organs concerned with the closing 

 of the pores and oscula, they are found to consist of rings of elongated 

 contractile cells or myocytes, which surround these apertures. These 

 rings of cells form veritable sphincters and their action is often efficient 

 enough to bring about a complete temporary closure of the aperture. 

 Whether the pores and oscula open by the counteraction of radial, con- 

 tractile myocytes or by the simple elasticity of the surrounding tissue 

 does not seem to have been determined. 



Since these sphincters lie very close to the epithelium that bounds 

 the surfaces of the pores or oscula and in fact probably often form a 

 part of this very epithelium, and since no nervous mechanism is known 

 to be connected with them, it seems very probable that they are brought 

 into action by direct stimulation and that the sponge is a metazoan in 

 which there are functional effectors unassociated with receptors or 

 adjusters. Thus the sponge would represent the first stage in the 

 differentiation of a neuromuscular mechanism, i. e., one in which the 

 effector in the form of a primitive muscle-cell is the only element 

 present. In my opinion it is around these contractile cells that the 

 nervous organs of the higher metazoans have developed and I therefore 



