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