skip to content

Darwin Correspondence Project

From C. G. Semper   13 July 1877


13th July 1877.

Dear Sir

It gives me the greatest pleasure that I am able to send You before leaving for America a copy of my work on the dorsal eyes of Onchidium, of which you have been so kind to accept the dedication.1

You will allow me to give here a postscript to the work.

On the authority of Dr. Bergh I stated, that in some Dorides (Sphaerodoris) existed black spots on the dorsum, which might be dorsal eyes.2 Since then, I have received from Bergh two species of the genus; but the black spots are only pigment spots without the slightest trace of eyes in them. You will see, that this result is a strong argument in favour of my hypothetical view, that the eyes have originated independently in the genus Onchidium, no other molluscs being known to possess the same or similar organs.

I have tried to explain the existence of these dorsal eyes by the relation, which exists between the Onchidium and the Periophthalmus, which preys on them.3 The latter is only to be found, where there exist Onchidium; the only exception being the existence of the fish at the West Coast of Africa, from where no Onchidium have been described till now, as far as I know. This exception is [rare], as I know now; the African Expedition of Berlin has brought home two or three species of these snails from Western Africa. It remains only to be seen, whether they have dorsal eyes or not. Those Onchidium, which live in places where no Periophthalmus hunts them—f.i. at South Australia, New Zealand, Galapagos, N. America and Europe (Onchidium celticum)4—are blind on the back.

I remain Dear Sir | Devotedly Yours | C. Semper.


See letter from C. G. Semper, 26 April 1877 and n. 2, and letter to C. G. Semper, 30 April 1877. The work was Über Sehorgane von Typus der Wirbelthieraugen auf dem Rücken von Schnecken (On visual organs of the vertebrate eye type on the backs of slugs; Semper 1877b). Onchidium is a genus of air-breathing sea slugs.
See Semper 1877b, p. 2 and n. Semper cited Rudolph Bergh. Sphaerodoris is a synonym of Actinocyclus, a member of the superfamily Doridoidea.
Periophthalmus is a genus of mud skippers. See Semper 1877b, p. 32.
Onchidium celticum is a synonym of Onchidella celtica, the Celtic sea slug.


Sends work on dorsal eyes of Onchidium ["Über Schneckenaugen", Arch. Mikrosk. Anat. 14 (1877): 118–24]. Comments on work.

Letter details

Letter no.
Carl Gottfried Semper
Charles Robert Darwin
Sent from
Source of text
DAR 177: 138
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 11050,” accessed on 16 October 2021,