On the eyes of polychaetes

Abstract

1. A detailed study of the eyes of a few common polychaetes of Madras with different habits has been carried out giving a brief comparison of their structure with that of allied forms from European waters. 2. The eye in Eunicidae is usually composed of a retinal cup and a central refractive body which consists of a localised thickening of the cuticle; the connection between the cuticle and the refractive body being retained in the adult worms. 3. In Nereidae which includes mostly free living errant forms, the refractive part of the eye consists of a more or less semi-solid glassy body which does not show any connection with the cuticle. 4. In the Tomopteridae which are pelagic the retina is a simple pigment cup without differentiation into retinal cells or rods. The nature of the refractive body is also very simple. 5. In Sabellids which are tubicolous the visual organs are of the nature of compound eyes occurring on the main stem of the branchiae. Each of these is composed of a number of individual units with a separate refractive body. 6. Although polychaetes of the same family have almost the same ground plan in the construction of their eyes, differences in the degree of development of the eyes have been observed within the same family. The causes of these differences in the visual organs have been attributed to the differences in the mode of life of the animals and it has been found that the free living or errant forms have much better developed eyes compared with the burrowing ones where the eyes are usually absent. In all of them the early free swimming larvae possess eyes but in the case of burrowing forms the eyes disappear as they grow, often leaving no trace of their existence in the adult condition.