My dear Sir

Although I have not yet received the Box with the Cirripedia, I cannot wait longer to send you & D^r. Gould my most cordial thanks for your great kindness.—² I hope that you will give my especial acknowledgments to D^r. Gould for the very great assistance, which the named specimens will afford me. I have been particularly gratified in receiving specimens from yourself, in as much as, when doubting whether to undertake a monograph of the class, or to confine myself to their anatomy, your sentence that “a monograph on the Cirripedia was a pressing desideratum in Zoology”³ much helped to decide me.— In your letter to M^r Lowell⁴ (to whom I also beg you to present my respectful thanks) you say you sh^d like to hear what results I have arrived at; as far as the limits of a letter go, it will give me real pleasure to do so.— I sh^d be glad if you would not mention my present results, partly because I sh^d like to have the satisfaction of publishing myself what few new points I have found out, & partly because one is more free to alter ones own views, when they are confined to one’s own breast.—

The Cirripedia are true Crustacea with no affinities to other classes; M. St. Ange⁵ curiously mistook a strong epithelium for an inner sort of stomach, & hence the affinity with the Annelides disappears. The Cirripedia are either a sub-class equal to the Siphonostomous & Maxillated classes &c or to all these classes together; this I cannot decide yet.⁶ All cirripedes (except one forming an abnormal order) in their first larval stage, have 3 pair of legs & two pair of antennæ in process of formation within cases, & one eye: in the last stage, all have six legs, two compound, not pedunculated, most singular eyes & a pair of prehensile antennæ. In this stage they are pupæ, for they have no mouth. In assuming the mature form, the compound eyes are moulted, & by the act of moulting (by a process which I cannot explain without diagrams) the cirripede assumes the position which it always holds, which is at right angles to that which the pupa assumed when first attached. I am particularly obliged for the information about D^r. Leidy. I know the black spots which he alludes to, but I had not discovered that they were eyes,⁷ though I thought them like eyes. I will attend to this. I have long been aware that the Cirripedia are exquisitely sensitive of light or rather shadows. Cirripedes have olfactory & auditory organs; the nerves proceeding from the infra œsoph. ganglion.—⁸

The means by which Cirripedes are attached is one of the most remarkable parts of their Natural History:⁹ a portion of the ovarian cæca, becomes modified & glandular & secretes a cement, which is poured out by 2 ducts opening at the penultimate segment of the antennæ of the pupa, & fixing them permanently to the support. Afterwards the cement in some families is poured out by symmetrical orifices round the base of shell or peduncle, or in a single line, when the cirripede is attached to a coralline.— When the pupa moults, the whole of its envelopes are moulted, except the cemented antennæ: hence by care (& extreme care is requisite) the antennæ of the pupa can be demonstrated in the exact centre of the base of the largest Balanus or Anatifera.—

Strange as it may appear, this curious means of attachment is the only character absolutely universal in the Cirripedia, (some Cirripedia are apodal ¹⁰ ).— I have made out, I believe with certainty from dissection & not from analogies the homologies of the cirripedia¹¹ The shell, including the peduncle of the Pedunculata, consists of the 3 anterior segments of the head, so enlarged as to receive the whole rest of body, which consists of 14 segments in the larva & in some abnormal cirripedia (the cirri answer to the 5 pair of ambulatory legs & the outer pied-machoire of the higher crustacea), the 4 posterior abdominal segments being always aborted.—

The great majority of cirripedes are bisexual,¹² but it seems that they can fecundate each other, for I have scrupulously examined a Balanus, which had had its penis cut off & was imperforate, but in which the ova were impregnated.¹³ I have one genus, in which the sexes are distinct, the male (sometimes two of them) being parasitic & permanently attached within the so-called sack of the female.—¹⁴ There is, however, one much stranger case, but of the accuracy of which I am positive after repeated dissections: the normal form is bisexual, but the male organs are small, though perfect & containing zoosperms, to make up for this, some of the pupæ, become attached to the orifice of the sack of the hermaphrodite, undergo a retrograde metamorphosis, never acquire a mouth or stomach, & are filled with testes, & ultimately the whole animal consists of a great sperm-receptacle full of zoosperms.¹⁵ As soon as these are discharged, these embryonic “supplemental males” die & are soon succeeded by another set: I have counted 10 of these supplemental males on one hermaphrodite. You will laugh at this account, but I assure you I would not presume to tell you anything, of which I was not sure, from repeated examinations of specimens taken at different periods & from different countries.—

The most remarkable individual cirripede, which I have seen is naked, apodal, with a suctorial mouth, & parasitic in a double way within another cirripede.—¹⁶ I hope I have not wearied you much with these details: I sh^d. be proud if anything I could say, would interest you.—

With sincere respect & my cordial thanks for your & D^r Goulds kindness— Pray believe me | Yours faithfully & obliged | C. Darwin

I do not suppose I shall finish my Monograph for two or three years— my health allows me to work very little