1892-93-] The Shipworm. 6g 



noria are found to eat most rapidly between the bottom and 

 low-water mark, but above low-water the damage is not so 

 great, and they do not appear to exist at all below the bottom. 

 Another opinion is to the effect that timbers which are alter- 

 nately wet and dry are destroyed much faster than those 

 constantly immersed. 



The small shells constituting the formidable boring appa- 

 ratus by which the long tubular burrows are excavated are 

 peculiarly fitted for the performance of this special duty. 

 Examination under the microscope reveals the fact that the 

 very fine lines on the shell, seen by the naked eye, and of 

 which there are two sets running at right angles in each 

 valve, resolve themselves into rows of teeth. On the broader 

 part they are serrated, while on the narrow part the teeth are 

 larger, somewhat rounded in shape, and slightly separated from 

 each other, but there are exactly the same number of rows in 

 the broad as in the narrow part. These teeth cut in a back- 

 ward direction. This also is evident by examination, and 

 even by touch. Although direct observation of the animal 

 in the act of boring has perhaps never been enjoyed, it appears 

 to me that the cutting lines of these four sets of teeth must 

 cross each other at two different angles, and, coupled with the 

 ceaseless circular or half-circular motion which must of neces- 

 sity go on, and the constant renewal of the lines of teeth from 

 behind as these are worn away in front, it is enabled to rasp 

 away very hard timber ; and the wonder increases rather 

 than lessens, how, by tools so insignificant, so fragile, and a 

 muscular force so trifling, such disastrous and terrible results 

 ensue. 



The borings, after an entrance has been effected, are gener- 

 ally made in the direction of the grain, except when a knot is 

 met with, which, being evidently too much for them, they go 

 round about. They usually occur in considerable numbers, 

 often crowded together, so that but a very thin partition 

 separates the adjacent burrows. They further appear to have 

 the faculty of knowing when they are boring into the tube of 

 a neighbour, or if the plank be thin that they are coming- 

 through altogether. From the specimens it will be seen that 

 the bore in such cases forms a small cul-de-sac, and that the 

 Teredo has withdrawn a little and begun afresh. The cavity 



