156 TEREDINID^. 



lour. A similar retention of embryonic parts occurs in 

 the case of beetles, the grubs of which do not part with 

 their horny jaws when they attain an adult state. It is 

 otherwise with the Lepidoptera, which exchange their 

 larval mandibles for a suctorial proboscis. The meta- 

 morphosis of Teredo is not less wonderful than that 

 which takes place in the frog, insect, or polype. 



5. Structure of Shell. — The sculpture of the shell is 

 excessively complicated and delicate. Harting counted 

 4000 denticles in the anterior portion, and nearly 10,000 

 in the middle division of a single valve of T. navalis. 

 Dr. Carpenter kindly examined, at my request, the 

 microscopical structure of the valves and sheath of T. 

 Stutchburii. He informs me that the valves are ex- 

 tremely hard in texture, and that their substance has a 

 very peculiar arrangement, corresponding generally with 

 that of the shells of the bivalves most nearly allied to it, 

 but having so special an adaptation to produce a fine 

 file-like disposition of the surface, that he cannot help 

 surmising there is more in the mechanical theory than 

 I am disposed to admit. The sheath is destitute of any- 

 thing like true structure, but has all the characters of a 

 mere exudation shell, formed of minute calcareous 

 particles, agglutinated together, very much like some 

 egg-shells. He adds that the difference in texture 

 between the two is nearly the same as that between the 

 half chalky substance of a crab's carapace, and the 

 almost ivory-like consistence of the black tips of its 

 claws. I would observe that the sheath of Kuphus 

 arenarius is remarkably solid and compact, with a 

 radiating structure, and that the surface of the shells in 

 some of the Pholadid(e, and even in species of Tellina 

 and other genera, exhibit a file-like arrangement. 



6. Origin. — An erroneous notion was formerly preva- 



