AN EXCEPTION TO BATESON'S RULE OF SECOND- 

 ARY SYMMETRY. 



A. B. DAWSON, 

 DEPARTMENT OF ANATOMY, LOYOLA UNIVERSITY SCHOOL OF MEDICINE. 



In the summer of 1917, while employed by the Biological 

 Board of Canada in a Dominion Lobster Hatchery at Bay View, 

 N. S., my attention was called to an abnormal lobster caught by a 

 local fisherman in the adjoining waters of Pictou Harbor. The 

 lobster was a male and measured 7>^ inches from rostrum to 

 telson. The abnormality consisted of a double extra claw on 

 the right cheliped, resulting in a condition of incomplete tripli- 

 cation. The presence of the extra parts on the right side did not 

 appear to greatly handicap the animal which, when placed in 

 one of the hatchery tanks, moved about freely. Other abnor- 

 malities somewhat similar have been described for the lobster 

 by Faxon ('81), Emmel ('07) and Cole ('10). 



DESCRIPTION. 



The "triple" chela of the right side consisted of a small "nip- 

 per" and a double extra "crusher," while the corresponding 

 appendage of the left side was large, normally developed and of 

 the nipping type. The small "nipper" of the monster claw 

 apparently represented the primary member of the group and, 

 with the exception of the meropodite (M.), was normal in all its 

 segments. Arising from the posterior (morphologically ventral) 

 surface of this meropodite was the double extra claw. 



The first segment of the right chela, the ischiopodite (Is.} 

 was not exceptionally large and presented no evidence of dis- 

 tortion; but in the meropodite, as already noted, the effect of the 

 abnormality in the appendage was very evident. This segment 

 was of normal width proximally but broadened out rapidly 

 distally and terminated in two diverging branches (M., M.' + ") 

 On M was borne the small primary claw while M.' + " carried 

 the much larger abnormal structure. 



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