The infrared sensory membranes of the pit organs of pit vipers have an extremely rich capillary vasculature, which has been noted passim in the literature, but never illustrated or studied in detail. We rendered the pit vasculature visible in various ways, namely, by microinjection of India ink, by a combination of ink and succinate dehydroge-nase staining, and by making resin casts for scanning electron microscope study. We also used transmission electron microscopy for identifying the types (arterioles, venules, capillaries) of blood vessels. Then we compared the pit vasculature with that of the retina and the dermis. Good visualization of the vasculature was obtained with both ink and resin injection. Arterioles, venules, and capillaries could be distinguished with all methods used. The monolayer vasculature was denser in the pit membrane than in the retina or skin. Each loop of the network enclosed a small number of infrared receptors so that all receptors were in contact with a capillary on at least one side. The forward-looking areas of the pit had a denser network than side-looking areas. Since infrared rays cause nerve impulses by raising the temperature of individual receptors, the capillary network functions not only as a supplier of energy but also as a cooling mechanism to reduce afterimages. Thus the denser network in the forward-looking areas causes these areas to be more sensitive and have better image resolution than the rest of the membrane.

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The surface of the epithelium in snake infrared receptor organs is covered with a characteristic array of tiny pores that is different from any other surface structure in squamate reptiles. The measurements and density of the pores differ slightly according to family and species, but the array is characteristic and immediately recognizable. In boids without pits, the array covers the entire surface of each scale that contains infrared receptors. In boids with pits, the array covers the fundus of each receptor pit organ. In crotaline pit organs the array is present on both the outer and inner surfaces of the receptor-containing membrane, and on the epithelium of the wall of the inner chamber. This inner chamber Wall is sculpted into a tight array of large and small domed structures, on the surface of which the pore array appears. We speculate that the array of domes in the crotaline pit organ functions as a light trap to prevent infrared rays that penetrate into the inner chamber from being reflected back onto the receptors in the pit membrane. On the other hand, the array of pores, present in all species, appears to reflect away and diffuse visible radiation that might have enough energy to heat-stimulate the receptors and interfere with the target stimulus, i.e., infrared radiation.infrared reception, and were found only in regions associated with the infrared receptors, we surmised that they played a major role in the receptor func-

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The hagfish lateral line system was studied by horseradish peroxidase transganglionic transport. The anterior lateral line nerve innervates the group of lateral line canals situated anteriorly to the eye, and the posterior lateral line nerve innervates the group of canals situated posteriorly to the eye. Although both nerves pass through the muscle fascia at the same point, each runs a different course to the brain. The anterior lateral line nerve runs near the trigeminal nerve and its ganglion is closely attached to the trigeminal ganglion, but both systems are completely independent. The posterior lateral line nerve runs independently of any other cranial nerve and makes a peculiar U-turn at the point of entry to the brain capsule. The anterior lateral line ganglion contains both cutaneous sensory cells (small to large cells) and lateral line sensory cells (small cells); from this ganglion projections run to both the trigeminal sensory nucleus (fine and thick fibers) and medial nucleus of the area acousticolateralis (fine fibers). The posterior lateral line ganglion contains only small lateral line cells that project fine fibers to the medial nucleus of the area acousticolateralis. There are no efferent components in this lateral line system, and its only afferent terminal field is the medial nucleus of the area acousticolateralis.