DISPERSAL BY MAN. 221 



one end of the canal to the other, a distance of sixteen 

 miles/ 



Planorbis DILATATUS Gould. This American mol- 

 lusc was first detected in the British Isles, and indeed in 

 Europe, during the summer of 1869, in the Bolton 

 Canal, at Pendleton, on the west side of Manchester, 

 and in the autumn of the same year it was discovered 

 also in the Gorton Canal, on the east side of the city, 

 the two localities being about five miles apart. In both 

 cases, the shells were found close to cotton-mills, 

 where the waste from the cleaning or blowing machines 

 was ejected over and about the canals — a good deal 

 finding its way into the water — and where the 

 warm water from the engines was discharged. Mr. 

 Thomas Rogers, the discoverer, and Mr. Jeffreys, who 

 determined the species, suggested that the animal, or 

 its eggs, had been imported from America with cotton, 

 and it was pointed out that the best cotton is culti- 

 vated in river-bottoms, and that the crop, when picked, 

 is spread out to dry. We have already seen that many 

 molluscs possess great tenacity of life, and, as mentioned 

 by Mr. Jeffreys, certain species oi Planorbis, in times of 

 adversity, close the mouths of their shells with an 

 epiphragm, and thus protected they keep alive out of 

 water for weeks and even months ; so that it may be 

 assumed that in all probability living specimens of P. 

 dilatatus could survive a voyage from America to Eng- 

 land ; and after examining the waste from the machines, 



• R. Tate, " Land and Fresh-water Mollusks," 1866, p. 23. 



