1030 



CRUSTACEA. 



are two bent setge in place of the second from the base. In figures 37 

 and 38, the longer setse of the outer half of the organ correspond to 

 those in figure 22, 23. The joints (counting from the apex) from 

 which these setae proceed, are in — 



Fig. 21, 



joints 5 



8 



12 



17 



22 



« 22, 



" 4 



7 



11 



16-18 



21 



" 24, 



« 4 



7 



11 



17-18 



21 



20 



17 



13 



8 



3 



20 



17 



13 



6,7 



3 



19 



10 



12 



5, 6 



2 



or, counting from the base, 



Fig. 21, joints 



« 22, " 



« 24, " 



Figure 21 has twenty-four joints, and figure 22, but twenty-three : 

 the two organs, as seen by comparing, differ in the former having 

 four terminal joints, corresponding to three in the latter. Again, 

 figures 22, 24 (the former with twenty-three and the latter twenty- 

 two joints), differ in the second joint from base, being divided in one 

 species and not in the other. 



If, then, twenty-four be the normal number of joints, the variations 

 from perfect correspondence in the above arise from the obsolescence 

 (or rather, non-development) of one or more articulations. 



It does not follow necessarily, that whatever the number of joints, 

 the longer seta3 will correspond severally in position to the same 

 normal joints of the organ. On the contrary, the facts as far as 

 observed are otherwise, and tend to prove that the positions have 

 rather a relation to the length or organogenic nature of the organ. 

 Thus, in figures 32 to 36, the longer seta near the middle of the 

 antenna is situated on the eighth, ninth, tenth, or eleventh joint 

 (from apex), according to the species. The organ, in an important 

 sense, has the character of a unit in its development, and the position 

 of the seta has a relation to its unit character, rather than to parti- 

 cular joints in a normal antenna. 



It seemed natural, at first thought, that the position of the longer 

 setse, in figures 21, 22, 37, 38, &c, should indicate the limits of the 

 joints in the few-jointed antenna. Yet this is by no means the case. 

 Compare 37 and 38, left and right antennse of the same individual : 



