A comparative study of the structure and chemistry of the retinal photoreceptors of the eye is under continuous investigation in our laboratory. A part of this study is focused on the image-forming compound eyes of the arthropods, which include the arachnids, insects, and Crustacea (13-18). T h e present report is a study of the eye structure of Leptodora kindtii, a planktonic crustacean belonging to the order Cladocera which is found in m a n y fresh-water lakes of North America, Europe, and Asia. T h e general morphology and behavior of Leptodora kindtii were described more than a halfcentury ago by Gerschler (5). Leptodora is a relatively large organism (up to 18 m m in length), nearly all transparent, with one median, spherical eye. Because of its eye structure and various types of neurons (9), as well as its general behavior to light stimuli, Leptodora is of interest to us in studies of visual physiology.
M A T E R I A L S A N D M E T H O D SLeptodora kingtii were collected from a depth of 1 meter at the University of Pittsburgh Field Biology Laboratory, Lake Pymatunlng, Pennsylvania. For electron microscopy, the organisms were immediately darkadapted for 1 hour and then fixed for 30 minutes at room temperature with l per cent osmium tetroxide (OsO4) in lake water, which included 45 mg/mi of sucrose and was buffered with Veronal-acetate to pH 7.5. After fixation, the organisms were washed with distilled water, then dehydrated by a series of graded alcohols, infiltrated with Vestopal W, flat-embedded, and polmerized. To obtain a preferred orientation for sectioning, areas were cut from the embedded material with a jeweler's coping saw and remounted. Sections were cut through the crystalline cone and photorcceptor regions of the compound eye, using a glass knife on a Porter-Blum ultramlcrotome. All sections were examined with an RCA-EMU-3F microscope.To determine the reactions of Leptodora to polarized light, an experimental method similar to that used by Waterman (10) for Daphnia was adopted, in which a low voltage, 25 watt, tungsten filament bulb provided a vertical beam of white light which passed successively through a heat filter, a depolarizer, and adjustable polarizer, (Polaroid), linearly polarizing the light almost completely. The experimental vessel was a Petri dish 10 cm in diameter and 1 cm deep, painted black to eliminate any reflections from the glass. The light intensity on the surface of the dish was about 20 foot-candles. Active, swimming Leptodora (50 per dish) were focused on a photographic easel and irradiated for 5 seconds with polarized light, then photographed on a plate, 31/~ x 4 inches. This operation was followed by a dark period of 15 to 20 seconds, and then another exposure on a photographic plate was made. 95 such plates which had been exposed in this manner were developed. The negatives were enlarged by printing, and the plane of polarization was indicated on each print. To obtain the angle of polarization, a line was drawn through the long axis of each Leptodora with respect to ...
