TRICHOBRANCHL, 268 
The gills are similar to those of the lobsters, but reach 
the number of twenty-one on each side. 
In their fundamental structure the rock lobsters agree 
with the crayfishes; hence the plans of the two may be 
regarded as modifications of a plan common to both. 
To this end, the only considerable changes needful in 
the tribal plan of the crayfishes, are the substitution of 
simple for chelate terminations to the middle thoracic 
limbs and the suppression of the appendages of the first 
somite of the abdomen. 
Thus not only all the crayfishes, but all the lobsters 
and rock lobsters, different as they are in appearance, 
size, and habits of life, reveal to the morphologist un- 
mistakable signs of a fundamental unity of organization ; 
each is a comparatively simple variation of the general 
theme—the common plan. 
Even the branchie, which vary so much in number in 
different members of these groups, are constructed upon 
a uniform principle, and the ‘differences which they 
present are readily intelligible as the result of various 
modifications of one and the same primitive arrange- 
ment. 
In all, the gills are trichobranchie ; that is, each gill 
is somewhat like a bottle-brush, and presents a stem 
beset, more or less closely, with many series of bran- 
chial filaments. The largest number of complete bran- 
chie possessed by any of the Potamobide, Parastacide, 
Homaride, or Palinuride, is twenty-one on each side ; 
