To Albany Hancock 25 December 
Down Farnborough Kent
My dear Sir
As you have attended with such eminent success to the boring of animals into rocks,1 you will perhaps like to hear that I believe I now understand the boring of Lithotrya,2 thanks to the enclosed drawing [SYMBOL HERE] (which please return) sent me by Steenstrup without text.3 I suppose the same explanation is applicable to Arthrobalanus, & I shd. think Alcippe, (for the presence of the calcareous disc is not material to the change of place) but not, as far as I can see, to Clisia.—4
Since receiving this same Plate, I have had a good deal of rock, bored by L. dorsalis, given me,5 & I now find out for the first time the following important facts, (1)st that the animal bores to its full depth when young & afterwards only increases the diameter of its hole. 2d. that a cup is only formed when the animal has ceased boring to a greater depth, but that before a cup is formed, a succession of little discs exactly as represented are deposited on one side of the hole, each new one, at each fresh exuviation being placed th or th of inch or even more beneath that last formed; the disc or cup, as I was always certain, never itself being moved. 3d. The lowest disc is never at the bottom of the burrow, & this is faithfully represented in the Plate. Lastly the skin of the peduncle at this bottommost part, at first after each exuviation is studded with minute calcareous beads, which are soon fairly worn away; & the beads are succeeded by hard horny star-headed points which are also much worn away, before a new moult, so that there is good wearing agency. (N.B I found specimen with perfect coat underneath old coat nearly ready to moult, so no possible mistake.) I shd. have said that as soon as the animal begins to increase much in diameter the chain of little discs are of course all worn away, so that no trace is left in full-sized specimens.
In the drawing you will at once understand how the animal travels, by imagining a set of ghosts or exuvia attached to each of the little discs one above the other.— I have seen a rows of disc extending an inch in length. (The teeth on the valves and on the beads on the peduncle, with their exuviations sufficiently explain the mere increase in diameter of the burrow.)
I cannot explain in a letter how the discs are fixed; but it is as in all other Cirripedia, by a cement or tissue (for I hardly know which to call it), which primarily debouches at the penultimate segment of the prehensile antennæ of the larva (this cement is formed by a gland, strange to say, which is certainly part of the branching ovaria) & subsequently during life, in different cirripedia, either through these 2 same orifices, or out of two fresh or only one fresh aperture placed symmetrically or irregularly or again through numerous apertures placed in a regular circle; so that it is nothing unusual in Lithotrya for the discs to be fixed symmetrically in a straight line. In Scalpellum the peduncle is attached to the thin stem of the Coralline by apertures, through which the cement debouches, placed quite symmetrically in a straight row along the ventral side, a new one being opened at each exuviation.—
But I must stop, & not weary you.— I think the drawing will make you understand what I mean, better than my perhaps ill-expressed explanations.
I have not yet looked at Alcippe!6 But do not suppose that I undervalue your kindness in having sent me the specimens; but I have been working like a wretched slave at mere species & have many more months’ work, & till I have completed this slavery, I have not heart to begin work of interest, for I think I shd. never get courage to resume the drudgery of describing species, & making out synonyms. I hope this letter will not bore you—
Believe me my dear Sir, | Your’s sincerely | C. Darwin
P.S. The accompanying spec., of as I suppose, a Cliona,7 you can throw in the fire if of no interest to you.— from Northern part of Patagonia.
Discusses capacity of some cirripedes to bore into rock. Describes progress of his research.