CONCHIFERA. 45 



secreting a glutinous substance, the latter is drawn 

 out by means of the groove into slender threads, 

 which are attached by their tips to the rocks and 

 other foreign bodies. The grooved foot is then 

 withdrawn, and the thread presently hardens into 

 a very strong, highly elastic sort of silk, called 

 hyssus. It is by the aggregation of many such 

 threads that the common Mussel [Mytilus) moors 

 itself to the rocks and to its fellows. 



It is not only soft mud or sand that yields to 

 the burrowing efforts of the Conchifera. Not a 

 few among them bore tunnels into clay, wood, 

 chalk, shale, sandstone, and even the most com- 

 pact marble. The mode in which this is performed 

 is not yet ascertained. It cannot be by a chemical 

 solvent secreted by the animal, since there is no 

 menstruum known which will act on substances so 

 varied. The constant rotation of the rough shell- 

 valves, the application of the end of the foot, which 

 in some cases is said to be studded with flinty 

 points, and the incessant projection of ciliary cur- 

 rents, wearing away the substance atom by atom, 

 have all been suggested, and are each maintained, 

 as the effective modus operandi^ by names of re- 

 putation. Perhaps all of these may be in turn 

 employed ; for no one of them seems to afford a 

 satisfactory solution of all the difficulties. 



With one or two doubtful exceptions, the Con- 

 chifera have the sexes distinct, though, for the 

 most part, the distinction can be recognised only 

 by anatomical examination. The eggs are re- 

 ceived into one or other of the pairs of gill-leaves, 

 which become much swollen with their presence. 



The young are hatched before they leave the 

 parent, but present a form and structure totally 



