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 internal 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, t. 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
Page:Popular Science Monthly Volume 75.djvu/63
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