TEREDO. 137 



dismissing the chemical theory) , " Mechanical force 

 seems also scarcely probable or even possible ; for it is 

 not very evident how this can be employed whenever a 

 lateral opening is to be made in the side of the tunnel. 

 This opening is usually at some distance from the inner 

 or further end^ and its edges are often very sharply 

 defined. If force were required to be exerted, these 

 sharp edges would be a serious inconvenience to the 

 Teredo, whose body is bent at this point into often con- 

 siderably less than a right angle ; such angles occur 

 more than once in the same specimen.^^ The marks at 

 the extremity of the tunnel, when examined under a 

 microscope, resemble in miniature those which are left 

 in mowing a grass lawn with a scythe ; but they are 

 arranged in a circular manner, and are continuous. 

 These marks are very numerous and narrow ; they do 

 not correspond with the anterior and striated part of 

 the valves, which (although rounded) are never bent at 

 such an angle as would produce the sharp lines exhibited 

 on the eroded cavity of the wood. The notorious fact 

 that the valves are covered with an epidermis is evi- 

 dently a stumbling-block in the way of M. Cailliaud ; 

 because it would be difficult to understand why this slight 

 film is not rubbed off", if the valves are used in scraping 

 the wood. He endeavours, with considerable ingenuity, 

 to dispose of the difficulty by assuming that the epi- 

 dermis is only formed temporarily and provisionally, to 

 protect the valves from the efi'ect of the acid which the 

 Teredo employs in dissolving its sheath or outer case, 

 in order to make a new one. I am not aware that any 

 part of this assumption has been verified by observation. 

 M. Cailliaud was even unable to detect the presence of 

 any acid in Teredo, although he has given us a long list 

 of other mollusks which secrete it, including not only 



