Ɛpsilon

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

1
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).
2
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).
3
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.)
4
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).
5
Campanularia is another genus in the order Leptothecatae (see n. 1, above).

Please cite as “DCP-LETT-6975,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-6975