Derivatives of Cilia in the Distal Sense Cells of the Retina of Pecten

The scallop, Pecten, a bivalved mollusk, has approximately 100 brilliantly colored blue-green eyes a millimeter or so in diameter located along the free margins of the mantle. Eaz.h eye contains a double retina which is separated from the lens by a thin connective tissue septum. Just beneath the septum lies the distal retina with its two cell types: the distal sense cell (Dakin's (1910) terminology), which is associated with the distal ramus of the optic nerve, and the external interstitial supporting cell. The proximal retina (which will be described in a subsequent communication) is just beneath the distal retina, and it contains rod cells associated with the proximal ramus of the optic nerve and internal interstitial supporting cells. This note describes observations concerning the fine structure of appendages on the specially differentiated septal border of the distal sense cells. Melhods Eyes of freshly dredged Pecten irradian# were excised and fixed for 60 minutes at 4°C. in an aqueous solution: 4 per cent OsO~, 2 per cent KsCrsOT, 0.2 per cent KOH, and 0.810 M sucrose. The eyes were dehydrated in ethyl alcohol and embedded in a 4:1 mixture of butyl and methyl methacrylate, sectioned with a Porter-Blum mierotome, and examined with a Philips EM-100A electron microscope. OBSERVATIONS Along the septal border of each distal sense cell are a small number of oval structures a micron or so in diameter. Fig. 1 is an electron micrograph depicting one such structure. I t is composed of alternate lightly and densely stained bands which are arranged concentrically and show gradations in spacing. These bands originate in the basal bodies of cilia, as can be seen in favorably oriented sections (such as Fig. 2). Rows of both basal bodies and ciliary stalks are seen on the bottom half of Fig. 2 and on the left side of Fig. 1. That these structures are ciliary basal bodies and stalks is illustrated by Fig. 3, which is an electron micrograph of one such stalk in transverse section showing nine dense loci, some of which appear to be double, arranged around the circumference. This corresponds closely to the structure of cilia in animals from diverse phyla which have been examined by Fawcett and Porter (1954) and others. In Fig. 2 ciliary stalks in longitudinal section (indicated by lines parallel to the stalks) are seen to be continuous with the bands. The entire concentrically banded structure, connected as …