UK) NERVOUS SYSTEM. 



when examined under the microscope. The sympathetics are 

 distinct along the aorta in the nereides, and the globular par- 

 ticles are seen in their neurilema. The ventral columns are 

 separate from each other in the sabelke. Where the body of 

 the annelides is much developed transversely, as in the leech, 

 the sea-mouse, the pectinaria, and many others, the ganglia 

 along the columns, and the columns themselves are much more 

 developed and distinct, and are visible at a much earlier period 

 in the embryo. The abdominal ganglia of the leech, as shown by 

 Weber, are quite obvious in the embryo of that worm while it is 

 yet in the ovum. Where the setae for progressive motion are 

 large, numerous, and moved by strong extensor and retractor 

 muscles, as in the pleione and many of the tubicolous anne- 

 lides, the nervous columns and their ganglia are also large 

 and distinct. The motor and sensitive nerves come generally 

 from the same parts of the columns. The distribution of the 

 minutest filaments are seen without dissection through the 

 transparent and colourless body of the common pectinaria, 

 and in most annelides the abdominal columns and ganglia, of 

 a white colour and firm consistence, are obvious to the naked 

 eye through the thin parietes beneath them. In some of the 

 broad naked apodal annelides, where the segments of the 

 body are very numerous and short, to facilitate their ser- 

 pentine movements, as in the leech, the abdominal ganglia 

 could not be safely accommodated in every ring of the trunk, 

 and we find but one ganglion, of considerable size, for every 

 three or four segments. These abdominal ganglia, about twen- 

 ty-five in the leech, are more closely approximated to each 

 other at the two ends of the nervous columns than in the 

 middle, which already indicates the commencement of that 

 longitudinal concentration of the ganglia so remarkable in the 

 higher articulata, as the almost constant approximation of 

 the lateral columns themselves on the median plain of the 

 abdomen indicates development in a transverse direction, and 

 a higher grade of this system than the detached condition 

 of the columns seen in the lowest helminthoid animals. 



The nervous system of the entomoid classes presents only 

 a more developed condition of the same plan of structure 

 presented by this system in the worms, and that of the most 

 elevated insect or crab begins its development with the 

 simplest helminthoid form, as the nervous system of man 



