Holochroal eye

A type of compound eye found in trilobites

Holochroal Eye

The holochroal eye is a type of compound eye found in trilobites, an extinct group of marine arthropods. This eye structure is characterized by its numerous small lenses, which are closely packed together and covered by a single corneal layer. Holochroal eyes are one of the three main types of trilobite eyes, the others being schizochroal eyes and abathochroal eyes.

Holochroal eye of a trilobite

Structure

Holochroal eyes consist of thousands of tiny lenses, each typically less than 0.1 mm in diameter. These lenses are made of calcite, a crystalline form of calcium carbonate, which is a common feature in the exoskeletons of trilobites. The lenses are arranged in a hexagonal pattern, allowing for a wide field of view. Unlike schizochroal eyes, holochroal eyes do not have individual corneal coverings for each lens; instead, a single corneal layer covers the entire eye.

Function

The holochroal eye provided trilobites with a broad and detailed view of their surroundings, which was crucial for survival in the diverse marine environments they inhabited. The compound nature of the eye allowed for excellent motion detection, which would have been advantageous for avoiding predators and locating prey. The calcite lenses also helped to focus light, although the resolution was not as high as that of schizochroal eyes.

Evolution

Holochroal eyes are considered to be the most primitive type of trilobite eye, appearing in the earliest trilobites from the Cambrian period. Over time, some trilobite lineages evolved more complex eye structures, such as the schizochroal eye, which featured larger lenses and individual corneal coverings. The evolution of these eye types reflects the diverse ecological niches that trilobites occupied and their adaptation to different environmental conditions.

Related Pages

• Trilobite
• Schizochroal eye
• Abathochroal eye
• Compound eye