Naturalization of Dreissena polymorpha. 363 



it has been found of a large size by Dr. Lloyd of that place. 

 But though the Learn itself is not navigable, yet it is in the 

 immediate vicinity of the canal, from which the Dreissena 

 has probably been introduced. I have further to add that 

 this shell has lately been planted by Mr. Stuchbury of Bris- 

 tol, in some of the waters near that place. 



It appears desirable to record these particulars, because it 

 may interest some of our field-naturalists to watch the gradu- 

 al spread of this species over the kingdom. Its propagation is 

 so astonishingly rapid, that it will probably become, in a few 

 years, one of our commonest British shells. 



I lately kept some of these molluscs, of different ages, alive 

 for some time in a basin of water. The full-grown individu- 

 als, though they had been torn from their native bed, soon se- 

 creted a fresh byssus, and became anchored to the bottom of the 

 basin. It is evident from this that the byssus, when first se- 

 creted, must be in a highly glutinous state, which enables it 

 to become attached to the smooth surface of glazed earthen- 

 ware. The young individuals still retained the power of lo- 

 comotion, and crawled like gasteropods over the bottom. — 

 They effected this by protruding their foot in advance of the 

 anterior or cardinal end of the shell, and advanced by alter- 

 nate expansion and contraction of this foot, dragging the shell 

 after them. They indulged their wandering propensities for 

 a few weeks, and then wisely followed the example of their 

 parents, by selecting some convenient nook, to which they 

 attached themselves contentedly for life. 



I further remarked that these molluscs, acephalous though 

 they be, have still an evident perception of light. When in 

 a quiescent state, they kept the shell partly open, with the 

 siphuncular and branchial apertures exposed ; but if any ob- 

 ject was suddenly brought over them, they immediately re- 

 ceded, and partially closed their valves, although care was 

 taken that no concussion should be given to the basin. 



I have only to add that I have examined numerous speci- 

 mens, without detecting any of the crystals mentioned by Mr. 

 Sowerby, 'Mag. Nat. Hist.' vol. ix., p. 643, and figured p. 573. 

 Irregular pearly Concretions are, however, of frequent occur- 

 rence. 



Evesham, June ISih, 1838. 



