TRICHOBEANCHI^. 263 



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 aU 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 branchiae, 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 aU, the gills are trichobranchicB ; 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- 

 chia possessed by any of the Potamobiidce, Parastacidce, 

 Homaridce, or PalinuridcB, is twenty-one on each side ; 



