Meristic Variation in Trochus Zizyphinus

Abstract

ON recently examining a number of specimens of Trochus zizyphinus collected at Plymouth in September 1900, it was noticed that one specimen exhibited a peculiar abnormality, viz. the presence of two supernumerary eyes on the right side (Fig. 1). On the left side of the animal both cephalic tentacle and ocular peduncle were perfectly normal. The right cephalic tentacle was also normal, and the ocular peduncle of this side, though bearing three eyes, presented only a slight furrow indicating a partial division between the original eye and the two which are secondary and supernumerary (Fig. 2). Several cases of supernumerary eyes in Gasteropods have already been recorded, and in some cases (for example, Patella, Littorina) duplication of the eye is accompanied by duplication of the cephalic tentacle. Double eyes have also been recorded in Helix, Clausilia, Phidiana, Murex, and Sub-emarginula¹; in the latter, supernumerary eyes were found on both right and left sides, though in the majority of other cases they were present on one side only. It would thus appear that only double eyes have been so far recorded, and that the presence of three eyes on the right side of this abnormal specimen of Trochus is, apparently, unique. All three eyes are perfectly formed, each being provided with crystalline lens, retina, and optic nerve, thus all of them were, in all probability, functional during life.