426 MOLLUSCA. 



those whose construction we are well acquainted with ; yet we may 

 not know what they are. For instance, the ink of the cuttle-fish may 

 he supposed to be the urine : if it is not, then most probable they have 

 no kidneys ; but as it is so unlike the urine in other animals, it would 

 never be suspected to be such, until the knowledge of their having no 

 such bodies as kidneys, similar to those in other a nim als, was obtained, 

 and no other animal having ' ink ; ' therefore it is reasonable to suppose 

 it is the urine 1 . 



[Class Lamellibranchiata.] 



The Ship- Worm [Teredo navalis, Linn.]. 



This worm eats a canal for itself in the outer plank of a ship : it is 

 a small animal when compared with its length. They are found of 

 very different lengths ; but, of whatever length the animal may be, the 

 canal in which it is found is always of the same length ; therefore its 

 tail is in all cases to be found at the external surface of the wood 

 where it first began, and its head at the other end of the canal. 



As it grows in length, it eats its way into the wood, and lengthens 

 its canal ; its increase in thickness is but small in proportion to that of 

 its length ; and that increase is always at the head ; so that the tail of 

 one an inch long is nearly as large as one two feet long. 



They in general bore in the direction of the wood : at first they go 

 obliquely inwards, and then they follow the grain; but this is not 

 constantly so : they are often put out of this direction by some other 

 worm in the same piece of wood ; for, whenever they come near the 

 canal of another, they then change their direction. There are often so 

 many of them together in the same piece of wood, that it shall be bored 

 so thick of holes'as it possibly can, without running into one another. 

 But they seldom or ever do go quite into another's canal : how they 

 avoid this is not easily ascertained. 



The canal is lined by a white shell, for it appears to have no con- 

 nexion with the body of the animal, although it is formed from it. It is 

 pretty thick and strong at the tail, becoming thinner and thinner 

 towards the head : it also grows larger. This increase of thickness is 

 owing to this part being first formed and still continuing to thicken 

 while the animal is alive, and the fore part of the animal while it grows 

 is, as it were, creeping out of it and extending the shell forwards, 

 which is at first thin. The canal in the wood is very smooth ; and, 

 where it terminates at the head, it is by a smooth concave surface, as if 

 drilled, or turned in a lathe. 



1 [See Art. ' Cephalopoda,' Cyclopaedia of Anatomy, vol. i. pp. 536. 540.] 



