204 SHELLS AND MOLLUSCOUS ANIMALS. 



marine animals, cannot exist in fresh water. A 

 species lives in the hardened mud on the shores 

 of the Indian seas, which, however, never bores 

 into wood. 



The shelly tube of our common ship-worm 

 varies in thickness in different circumstances. 

 Mr. John Mummery, of Dovor, who has made 

 many observations on the habits of the marine 

 animals of that coast, informs the writer that he 

 possesses specimens taken from a portion of an 

 old wrecked vessel, lying on the sandy shore at 

 the north of Deal, in which the calcareous tube is 

 scarcely thicker than lawn paper. " But, in a 

 specimen," adds this gentleman, " procured from a 

 jetty near the railway terminus at Dovor, the 

 wood- work of which jetty has been almost de- 

 stroyed in the course of a few years by the 

 teredo, the calcareous lining, when removed from 

 the wood, I find to resemble more nearly the shell 

 of a serpula than the ordinary coating secreted by 

 the former animal. It is nearly one-eighth of an 

 inch in thickness ; and I attribute the increase in 

 substance to the fact, that the water was always 

 turbid with the immense quantity of chalk sus- 

 pended in it, consequent upon the extensive masses 

 of cliff which were thrown into the sea by the 

 railway excavators. The animal being thus sup- 

 plied with unusually abundant material, was 

 enabled to secrete a solid crystalline and extremely 

 hard shell, instead of the ordinary fragile depo- 

 sition. I have similarly thick cases from the 

 tropics, where lime is much more abundantly 

 present, either in solution or suspension, as may 

 be inferred from the greater number of calcareous 

 corals in those seas than in ours." 



