Mr Mcintosh, On Variation in the Norway Lobster. 441 



On Variation in the Number and Arrangement of the Male 

 Genital Apertures and on the Proportion of the Sexes in the 

 Norway Lobster (Nephrops norvegicus). By D. C. McIntosh, M.A., 

 Aberdeen. (Communicated by Francis H. A. Marshall, M.A., 

 Christ's College.) 



[Received 29 February 1904.] 



While engaged last autumn in an investigation on the fauna 

 of the Clyde area, I had my attention drawn to the variation 

 in the number and arrangement of the genital apertures, and to 

 the relative proportion of the sexes, in the Norway Lobster. 



Mr F. H. A. Marshall has already considered these two points 

 and has published 1 the results of an examination of 1068 speci- 

 mens. All of these he procured from the Firth of Forth, where 

 they are much more numerous than on the West coast. The 

 main purpose of the present communication is to consider the 

 bearing of the facts ascertained by me upon Mr Marshall's results. 



In my investigation a total of 656 Norway Lobsters was ex- 

 amined, the specimens being dredged from the Firth of Clyde 

 during the first fortnight of September, 1903. 



1. As regards the male genital apertures. 



Mr Marshall found ten forms of variation in the arrangement 

 of the male genital apertures (in addition to the normal arrange- 

 ment of one opening at the base of each of the last pair of 

 walking legs). The percentage of abnormal specimens was as 

 high as 12"2. On the other hand the percentage of abnormality 

 in the Clyde specimens only amounted to 2 - 4 9. Table I, which 

 should be compared with that given by Mr Marshall, indicates 

 the different arrangements of spermatic apertures which I found 

 among the Clyde lobsters. The letters r and I denote the right 

 and left sides. 



Table I. 



1 P. Z. S. London, 1902. 



VOL. XII. PT. V. 



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