486 THE SENSORIAL FUNCTIONS. 



We again meet with traces of nervous filaments, 

 accompanied also with muscular bands of fibres, in 

 some of the more highly organized Entozoa. In 

 the Ascaris, or long round worm, a slender and ap- 

 parently 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 

 exhibiting 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 divisible into two, by careful manipula- 

 tion. In addition to this characteristic feature, 

 these cords present in their course a series of en- 

 largements, appearing like knots ; one pair of these 

 generally corresponding to each of the segments 

 of the body, and sending off, as from a centre, 

 branches in various directions : so that each seg- 

 ment of the body of articulated animals, of an 

 annular structure and cylindric form, such as the 

 long worms and the myriapoda, has in many re- 

 spects, an independent vital existence. When the 

 body is divided into two or more parts, each portion 

 appears also to retain both the faculty of sensation, 

 and the power of voluntary motion ; an appearance 

 which, for the reasons already stated, is probably 

 deceptive. As far as we can judge, the only ex- 

 ternal sense capable of being exercised by this 

 simple form of nervous system, is that of touch ; all 

 the higher senses evidently requiring a much more 

 developed and concentrated organization of nervous 

 ganglia. 



