68 LECTURE VI. 



Coincident with this higher development of the muscular sj'stem in 

 the coelelminthic Entozoa is the more obvious elimination of the nervous 

 filaments, which in the Linguatula radiate from a distinct suboeso- 

 phageal ganglion. Amongst the Nematoidea the great Strongylus is 

 a favourable subject for the demonstration of the nervous system. 



In the Strongylus gigas, a slender nervous ring surrounds the 

 beginning of the gullet^, and a single chord is continued from its 

 inferior part, and extends in a straight line along the middle of the 

 ventral aspect to the opposite extremity of the body, where a slight 

 swelling is formed immediately anterior to the anus, which is sur- 

 rounded by a loop analogous to that with which the nervous chord 

 commenced. The abdominal nerve is situated internal to the longi- 

 tudinal muscular fibres, and is easily distinguishable from them with 

 the naked eye by its whiter colour^ and the slender branches M'hich it 

 sends off on each side. These transverse twigs are given off at 

 pretty regular intervals of about half a line, and may be traced round 

 to nearly the opposite side of the body. The entire nervous chord 

 in the female of this species passes to the left side of the vulva, and 

 does not divide to give passage to the termination of the vagina, as 

 Cloquet describes the corresponding ventral chord to do in the 

 Ascaris hmibricoides. In the latter species, and most other Nema- 

 toidea, a dorsal nervous chord is continued from the oesophageal 

 ring down the middle line of that aspect of the body corresponding 

 to the ventral chord on the opposite aspect. 



In the Linguatula tcenioides a proportionally large ganglion is 

 situated immediately behind the mouth, and below the oesophagus : 

 small nerves radiate from this centre to supply the muscular apparatus 

 of the mouth and contiguous prehensile booklets ; and two large 

 chords pass backwards and extend along the sides of the abdominal 

 aspect of the body to near the posterior extremity, where they ex- 

 pand and are lost in the muscular tissue. I have already alluded to 

 the evidences of the nervous system afforded by the ocelli in the 

 young of some species of Trematoda, in the full-grown Polystoma of 

 the urinary bladder of the toad and frog, and in the PlanaricB, We 

 have as yet no evidence that any species of Ccelelmintha possesses 

 rudimental organs of vision, at any stage of existence. 



The digestive organs are very simple, and are subject to little 

 variety in the Nematoid worms ; an ample alimentary canal, sus- 

 pended to the parieties of an abdominal cavity, extends in nearly a 

 straight line from the mouth to the anus, which are at opposite ex- 

 tremities of the body. 



In the Filaria the mouth is a simple circular pore, sometimes 

 surrounded by a circle of radiated papillae ; a short and slender 



