178 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
crab is symmetrical. The supra-oesophageal ganglion resembles in 
its finer structure the same ganglion in Carcinus (Bethe, ’ 95 . ’ 97 ) 
or Astacus (Ivrieger, ’ 80 ). The ventral ganglia are somewhat con¬ 
centrated (pi. 7, fig. 29). Those that supply the mouth parts form 
a quadrate mass which is closely associated with the more posterior 
thoracic ganglia. The three interior of these are large with strong 
ventral projections of the fiber masses; the two posterior are 
insignificant. 
The abdominal portion of the nervous system is but little modi¬ 
fied by the asymmetry. The ganglion of the first segment is not 
apparent and is presumably fused with the thoracic mass (Bouvier, 
’ 89 ). The five remaining ganglia are distinct (pi. 7, fig. 30) and 
those for segments two, three, and four fire displaced to the left. 
The displacement of the ganglion for the fourth segment is very 
slight and scarcely establishable in the adult crab, but is w r ell shown 
in sections of adolescent crabs. Each abdominal ganglion gives off 
a pair of ganglionic nerves (gn) beneath the integumentary muscles, 
either to a pleopod or to the point where a pleopod morphologically 
ought to be situated. The areas supplied by these nerves also 
receive a more delicate commissural (comm ri) above the muscles 
from the commissure cephalad from the ganglion. A similar 
arrangement of sub- and supramuscular nerves is found in Gebici 
affinis. There is no obvious difference in the size of the nerves 
supplying the right and left sides of the body, and they all show 
fibers and bipolar cells. This lack of asymmetry appears less 
striking when we recollect that these nerves are probably very 
largely sensory and this function would be nearly equal on both 
sides of the body through the hairs on the pleopods on the right 
and the tufts of hair on the left at the points where pleopods 
ought to stand. The function of the muscles also is very simple 
and so the larger size of the right flexors cannot greatly disturb 
the equality of the two sides of the body with respect to other 
structures. 
The nervous system develops almost without metamorphosis. 
The earlier larval stages have a supra-oesophageal ganglion of 
essentially the same structure as that of the adult, but differing 
in the smaller relative bulk of the fiber masses and fiber tracts as 
compared with the ganglion cells. In the zoea phase these cells 
completely surround the fiber masses and tracts, and although less 
