710 ADDRESS TO THE DEPARTMENT OF ANATOMY 



Eimer, and more especially by the brothers Hertwig, of Jena. 

 Careful histological investigations, especially those of the last- 

 named authors, have made us acquainted with the forms of 

 some very primitive types of nervous system. In the common 

 sea-anemones there are, for instance, no organs of special sense, 

 and no definite central nervous system. There are, however, 

 scattered throughout the skin, and also throughout the lining of 

 the digestive tract, a number of specially modified epithelial 

 cells, which are no doubt delicate organs of sense. They are 

 provided at their free extremity with a long hair, and are pro- 

 longed on their inner side into a fine process which penetrates 

 the deeper part of the epithelial layer of the skin or digestive 

 wall. They eventually join a fine network of protoplasmic fibres 

 which forms a special layer immediately within the epithelium. 

 The fibres of this network are no doubt essentially nervous. In 

 addition to fibres there are, moreover, present in the network 

 cells of the same character as the multipolar ganglion-cells in 

 the nervous system of Vertebrates, and some of these cells are 

 characterized by sending a process into the superjacent epithelium. 

 Such cells are obviously epithelial cells in the act of becoming 

 nerve-cells ; and it is probable that the nerve-cells are, in 

 fact, sense-cells which have travelled inwards and lost their 

 epithelial character. 



There is every reason to think that the network just described 

 is not only continuous with the sense-cells in the epithelium, but 

 that it is also continuous with epithelial cells which are provided 

 with muscular prolongations. The nervous system thus consists 

 of a network of protoplasmic fibres, continuous on the one hand 

 with sense-cells in the epithelium, and on the other with muscular 

 cells. The nervous network is generally distributed both beneath 

 the epithelium of the skin and that of the digestive tract, but is 

 especially concentrated in the disc-like region between the mouth 

 and tentacles. The above observations have thrown a very clear 

 light on the characters of the nervous system at an early stage 

 of its evolution, but they leave unanswered the questions (i) 

 how the nervous network first arose, and (2) how its fibres 

 became continuous with muscles. It is probable that the nervous 

 network took its origin from processes of the sense-cells. The 

 processes of the different cells probably first met and then fused 



