214 THE BIOLOGY OF THE SEA-SHORE 



plaiycheles, and Munida rtigosa), which live on flat shores, 

 are able to guard against the choking of the branchial 

 chamber and the clogging of the gills by mud in suspension. 



What are undoubtedly the most careful and convincing 

 researches on this subject have been contributed by Welldon 

 (1899). They are indeed classic. Welldon's researches 

 were primarily directed towards estimating quantitatively 

 the effect of the continued selection of small variations in a 

 particular direction, but incidentally they throw consider- 

 able light on the subject we have under consideration. By 

 careful measurements extending over a period of six years 

 Welldon and Thompson established that the mean frontal 

 breadth of common shore crabs {Carcinus moenas) living in 

 Plymouth Sound was diminishing yearly at a rapid rate. 

 Welldon formed the opinion that the change had been 

 brought about by selective destruction of crabs due to 

 changes in the physical conditions of the Sound, or more 

 precisely, to a progressive increase in the turbidity of the 

 waters of this area as a result of certain alterations effected 

 by man. In particular, the construction of a huge breakwater 

 has led to an increase in the quantity of fine mud (china 

 clay from the adjoining land carried into estuaries on either 

 side of Plymouth) on the shores and bottom of the Sound, 

 since the mud can no longer be carried away by the scour 

 of the tide or the waves of severe storms. By means of 

 experiments conducted in the laboratory Welldon proved 

 that the effect of this mud when in suspension is to cause 

 the gills of some of the crabs to become choked, and the 

 greater the frontal breadth of a crab the more readily will 

 the choking occur. The gills of crabs which died during 

 an experiment with fine clay or mud suspended in the water 

 were covered with a fine white deposit which was not 

 found in the gills of the survivors. The reason for the 

 escape of crabs of narrow frontal breadth is because this 

 character renders one part of the process of filtration of the 

 water of respiration more efficient than in crabs in which 

 the frontal breadth is greater. 



These experiments of Welldon's are among the few which 



