Changes in the Dimensions of Carapace of Carcinus moenas. 195 



the gametophyte of the latter, with its reduced number of chromo- 

 somes, finds its analogue merely in the maturing sexual cells of Fucus. 

 But until we know more of the nuclear changes as they occur in other 

 Algae, and especially in the more primitive forms, it seems unadvis- 

 able to go further than to indicate the possibility that we may require 

 to revise our present ideas on the comparative morphology of the 

 higher and lower groups of the vegetable kingdom. Even if we regard 

 the reduction in the number of the chromosomes as a fact which is 

 primarily of physiological importance, we may safely conclude, from 

 the universality of its occurrence, that it is also intimately connected 

 with the phylenogenetic development of living forms, and hence it 

 must meet with due recognition on the part of the morphologist who 

 is engaged in comparing the life-history of one group of organisms 

 with that of others. 



" On certain Changes observed in the Dimensions of Parts 

 of the Carapace of Carcinus mcenas." By HERBERT 

 THOMPSON. Communicated by Professor W. F. R. WELDON- 

 F.R.S. Received May 19, Read June 11, 1896. 



In making some measurements of young male Carcinus mamas 

 from Plymouth, corresponding to those made by Professor Weldon 

 on young females of the same species, and published by him in the 

 Report of a Committee for conducting statistical inquiries into the 

 measurable characteristics of plants and animals (' Roy. Soc. Proc.,' 

 vol. 57, p. 360), some interesting facts were observed as to changes 

 taking place in the relative dimensions of certain parts of the 

 carapace of these crabs in the space of the last three years. 



The carapace of the adult male crab, measured in the median 

 antero-posterior line is, roughly, from 40 to 60 mm. long. Now, of 

 young male C. mcenas collected at random at Plymouth in the year 

 1893, I had, for the purposes of measurement, 3,077 specimens, 

 ranging between 10 and 15 mm. in length of carapace, and on these, 

 besides the carapace length, as above defined, two other measure- 

 ments were taken, viz. (1) "frontal breadth," the distance in a 

 straight line between the tips of the two teeth which form the outer 

 "boundaries of the orbits, and (2) the "right dentary margin," 

 measured in a straight line from the tip of the first to that of the 

 last lateral tooth on the right side of the carapace. 



The measurements were made in the way described in the Report 

 above mentioned (ibid., pp. 361 2) : and owing to the rapid growth 

 and alteration of proportional dimensions in the young crabs, they 

 were sorted into groups, the members of each of which differed by 

 less than 0'2 mm. in carapace length, thus giving five groups for 



