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Darwin Correspondence Project

From George Busk   10 September 1871

Taynuilt | Argyleshire

Sept 10th 1871

My dear Darwin

Your letter has re〈ac〉he〈d〉 me in this rem〈o〉te latitude or should have had an earlier reply.1

My notion with respect to the relationship between the vibracular & avicularian organs & the ordinary cells & their contents of the Polyzoon is conceived somehow in this sense—that the mandible or vibra〈cu〉lum as the case may be represents the moveable lip of the cheilostomatous cell-mouth and consequently that the av〈i〉cularian cell might be 〈re〉garded as homologous with that inhabited by the nutritive & reproductive zooid.2

I have never however noticed any like a transition from the zooid itself into an avicularian or vibracular organ, although I conceive it is quite probable that the contents of the avicularian cell—for instance, the brushlike organ, seen in some cases &c—may be homologous with the ord〈in〉ar〈y〉 zooid.3

As regards a transitio〈n〉 or gradation between 〈    〉 avicularia & vibra〈cula〉 I could were I at home furnish you with several instances, whose names I cannot at the moment recall—whilst the ordinary vibraculum maybe represented thus


Another will p〈r〉esent this form


and sometimes the beak is still more produced.4

With respect to the opposite or fixe〈d〉 beak—such a thing 〈is b〉y no means universally 〈pre〉sent even in well marked 〈vibr〉acula especially when 〈the m〉andible is rounded, so




that its presence or absence is not of high importance. In the larger & more perfect vibracula as in Scrupocellaria and Caberea5—the seta is received in a deep groove but suc〈h a〉 groove is not essential in all cases & in many instances, more especially in the Lepraliæ it would be impossible, when the moveable member is removed, as in fossil s〈p〉ecies to determine, in all cases, from the appearance of the o〈rgan〉 whether the organ had been furnished with a vibracul〈um〉 or a true mandible.—6

I think but am not sure that Smitt has made some remarks with regard to these organs pertinent to your queries— when I get home I will see & send you his pape〈r〉—7

Am Yours very truly | Geo Busk

CD annotations

4.3 vibraculum] underlined blue crayon; ‘avicularium??’ above, blue crayon
5.2 〈vibr〉acula] ‘〈avi〉cularia?’ above blue crayon


See letter to George Busk, 2 September [1871] and nn. 4 and 5. Busk probably uses ‘cheilostomatous’ as a synonym for ‘calcified’; the lips of the Cheilostomata (an order of marine Bryozoa) are calcified (see Busk 1852–4, 2: 62).
CD cites Busk for this information in Origin 6th ed., p. 193. What Busk describes as organs are now referred to as heterozooids or specialised zooids. Busk’s ‘zooid’ is an autozooid, that is, a feeding or common zooid, capable of carrying out all life functions in a monomorphic colony (heterozooids cannot do so).
Scrupocellaria and Caberea are both genera in the family Candidae, which is in the order Cheilostomata.
CD cites this observation in Origin 6th ed., p. 194 (see also letter to George Busk, 2 September [1871] and n. 5).
CD cites Fredrik Adam Smitt along with Busk and Hinrich Nitsche in Origin 6th ed., p. 193.


Busk, George. 1852–4. Catalogue of marine Polyzoa in the collection of the British Museum. 2 vols. London: printed by order of the Trustees.

Origin 6th ed.: The origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. 6th edition, with additions and corrections. By Charles Darwin. London: John Murray. 1872.


Replies to CD’s queries about the homologies of the avicularian and vibracular organs of Polyzoa and gives examples of gradations between the two.

Letter details

Letter no.
George Busk
Charles Robert Darwin
Sent from
Source of text
DAR 160: 383
Physical description
5pp damaged †

Please cite as

Darwin Correspondence Project, “Letter no. 7930,” accessed on 29 January 2020,

Also published in The Correspondence of Charles Darwin, vol. 19