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Darwin Correspondence Project

From George Busk   7 November 1869

32 Harley St

Nov 7 1869

My dear Darwin

The Polyzoan creatures you saw emerging from the cells of the main polypidom appear to me to be parasites— There can be no doubt that the principal growth as seen in Fig 1–5 &c is a species of Sertularia, though I am unable at the moment to give it a name if it have one.1

The vacated cells of this growth have been taken possession of by a minute species of ctenostomatous Polyzoa2—usually as it would seem or at any rate frequently in pairs as in Fig 3—but also in many cases singly—Fig 2–4— 6, 7. And it is remarkable that the parasite seems to be always affixed at about the same point in the empty sertularian cell (Fig 3 & 4). It is a curious, but as you well know not unexampled instance of “necessary parasitism”.3

Another instance though of a very different kind of this kind of parasitism among the polyzoa is that which I have described under the name of Bugula Ditrupæ which appears invariably to grow from the upper part of the shell of a species of Ditrupa in Madeira.4

Besides this “necessary” guest however—the sertularian is much infested with a Campanularia and a small sponge.5

Believe Me | Yours very truly | Geo Busk


The diagram Busk refers to has not been identified, and no letter of enquiry to Busk on this subject has been found. CD had been in London since 1 November (CD’s ‘Journal’, Appendix II), and had been been making observations at the British Museum and the Zoological Gardens, Regent’s Park. The former class Polyzoa included the present phyla Ectoprocta (bryozoans) and Endoprocta. Sertularia was classified by Busk as a genus of the family Sertularidae in the order Anthozoa Hydroida (Busk 1852, p. 387). It is now a genus in the order Leptothecatae in the class Hydrozoa of the phylum Cnidaria. The polypidom (now polyparium) is the common stem or supporting structure of a colonial invertebrate (OED).
Busk classified Ctenostomata as a suborder of the order Infundibulata in the class Polyzoa (Busk 1852, p. 349). It is now an order in the class Gymnolaemata (tubular bryozoans).
Busk describes what was probably a surficial (epizoic) growth of the ctenosome on the empty Sertularia cells, known as thecae; the association was unlikely to be parasitic. The term ‘necessary parasitism’ was not used by CD or others; Busk may have used the term because the ctenosomes attached to what appeared to be the only viable sites on the hydroid. (Dennis Gordon, personal communication.)
Busk refers to his description of Bugula ditrupae in Quarterly Journal of Microscopical Science 6 (1858): 261. Ditrupa is a genus of polychaete worm, now in the family Serpulidae (fanworms).
Campanularia is another genus in the order Leptothecatae (see n. 1, above).


Busk, George. 1852. Account of the Polyzoa, and Sertularian zoophytes, collected in the voyage of the Rattlesnake, on the coasts of Australia and the Louisiade Archipelago.In Narrative of the voyage of H. M. S. Rattlesnake commanded by the late Captain Owen Stanley during the years 1846–1850, including discoveries and surveys in New Guinea, the Louisiade Archipelago, etc., by John MacGillivray, vol 1. London: T. & W. Boone.

OED: The Oxford English dictionary. Being a corrected re-issue with an introduction, supplement and bibliography of a new English dictionary. Edited by James A. H. Murray, et al. 12 vols. and supplement. Oxford: Clarendon Press. 1970. A supplement to the Oxford English dictionary. 4 vols. Edited by R. W. Burchfield. Oxford: Clarendon Press. 1972–86. The Oxford English dictionary. 2d edition. 20 vols. Prepared by J. A. Simpson and E. S. C. Weiner. Oxford: Clarendon Press. 1989. Oxford English dictionary additional series. 3 vols. Edited by John Simpson et al. Oxford: Clarendon Press. 1993–7.


The Polyzoa CD saw emerging from cells of a particular growth appear to be parasites. The main growth is a species of Sertularia; the minute parasites take possession of vacated cells.

Letter details

Letter no.
George Busk
Charles Robert Darwin
Sent from
London, Harley St, 32
Source of text
DAR 160: 382
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 6975,” accessed on 23 January 2020,

Also published in The Correspondence of Charles Darwin, vol. 17