540 THE SENSORIAL FUNCTIONS. 



structure of muscular bands of fibres, in these 

 animalcules, is a further evidence of the con- 

 nexion which exists between nerves and muscles. 

 We again meet with traces of nervous fila- 

 ments, 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 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 exhibiting any 

 ganglionic enlargement. This may be consi- 

 dered as the first step towards the particular 

 form of the nervous system of the higher classes 

 of articulated animals, where the principal ner- 

 vous cord is obviously double throughout its 

 whole length, or, if partially united at difierent 

 points, it is always readily divisible into two, by 

 careful manipulation. In addition to this cha- 

 racteristic 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 centre, branches in various direc- 

 tions. It is probable that these knots, or ganglia, 

 perform, in each segment of the worm, an office 

 analogous to that of the brain and special mar- 

 row of vertebrated animals, serving as centres of 

 nervous, and perhaps also of sensorial powers. 



