380 THE SENSORIAL FUNCTIONS. 



the body. The discovery of a regular structure of muscu- 

 lar bands of fibres, in these animalcules, is a farther evi- 

 dence of the connexion which exists between nerves and 

 muscles. 



We again meet with traces of nervous filaments, accom- 

 panied also with muscular bands of fibres, in some of the 

 more highly organized Entozoa. In the ^scaris, or long 

 round worm, a slender and apparently single filament is seen 

 passing forwards, along the lower side of the abdomen, till it 

 reaches the oesophagus, where it splits into two branches, 

 one passing on each side of that tube, but without exhibit- 

 ing any ganglionic enlargement. This may be considered 

 as the first step towards the particular form of the nervous 

 system of the higher classes of articulated animals, where 

 the principal nervous cord is obviously double throughout 

 its whole length, or, if partially united at different points, it 

 is always readily divisible into two, by careful manipula- 

 tion. In addition to this characteristic feature, these cords 

 present, in their course, a series of enlargements, appearing 

 like knots; one pair of these generally corresponding to each 

 of the segments of the body, and sending off, as from a cen- 

 tre, branches in various directions. It is probable that these 

 knots, or ganglia, perform, in each segment of the worm, an 

 office analogous to that of the brain and spinal marrow of 

 vertebrated animals, serving as centres of nervous, and per- 

 haps, also, of sensorial powers. Many facts, indeed, tend to 

 show that each segment of the body of articulated animals, 

 of an annular structure and cylindric form, such as the long 

 worms and the myriapoda, has in many respects an inde- 

 pendent sensitive existence, so that when the body is di- 

 vided into two or more parts, each portion retains both the 

 faculty of sensation, and the power of voluntary motion. As 

 far as we can judge, however, the only external sense which 

 is capable of being exercised by this simple form of nervous 

 system, is that of touch; all the higher senses evidently re- 

 quiring a much more developed and concentrated organiza- 

 tion of nervous ganglia. 



