190 CRUSTACEA EUCARIDA — DECAPODA chap. 



the same thing, though not to the same extent, was happening 

 in female Crabs. Weldon supposed that this change might be 

 correlated with the silting up of Plymouth Sound and the 

 consequent fouling of the water. To test this hypothesis he 

 kept a very large number of male Crabs in water to which fine 

 porcelain clay was added and kept in continual motion. In the 

 course of the experiments the survivors and the dead were 

 measured, and it was found that the mean carapace-breadth of 

 the survivors was less than that of those that succumbed. The 

 experiment was repeated with the fine sand that is deposited 

 and left at low water upon the stones on Plymouth beach, and 

 the same result was observed. It was also noticed that the 

 individuals which died had their gills clogged with the sand, 

 while those that survived had not. As a further confirmation, 

 a great many young male Crabs were isolated and kept in pure 

 filtered water, and they were measured before and after moulting ; 

 these measurements, when compared with measurements of the 

 frontal breadth in Crabs of the same size taken at random upon 

 the beach, were found to show a greater breadth than the wild 

 Crabs, thus indicating that a selection of narrow Crabs was 

 taking place in Nature which did not take place when the 

 Crabs were protected from the effects of fine sand in the 

 water. 



The whole chain of evidence goes to show that the carapace 

 breadth in Carcinus maenas in Plymouth Sound is being influ- 

 enced by the rapid change of conditions occurring in the locality. 

 Various objections have been urged against this conclusion, but, 

 though they merit further investigation, they do not appear very 

 weighty. 



The fresh-water Crab, Thelpliusa fluviatilis, common in the 

 South of Europe and on the North coast of Africa, belongs to 

 the Cyclometopa, and is interesting from its direct mode of 

 development without metamorpjhosis. 



Fam. 1. Corystidae. — The orbits are formed, but, unlike all 

 the other families of the Cyclometopa, are incomplete. The 

 body is elongate and oval, and the rostrum and front edge of the 

 mouth rather as in the Oxyrhyncha, in which Tribe they are 

 sometimes included. Corystes, with a few species in European 

 seas. C. cassivelaunus at Plymouth. 



Fam. 2. Atelecyclidae. — Perhaps related to the foregoing. 



