THE RESPONSE TO LIGHT IN ICHTHYOPHIS (AMPHIBIA‐APODA) FROM CEYLON

Abstract

SUMMARY Specimens of the virtually blind apodan, Ichthyophis glutinosus, have been exposed to general and local illumination, their sensitivity to light confirmed, and the character and timing of the resulting movements analysed. To general illumination the animal usually gives a general stirring and locomotory resopnse after a considerable and variable delay. In some animals this general response is preceded by a shock reflex, the twitch response, the reaction times of which are reasonably consistent and show the intensity/duration relationships discovered by Hecht in automatic light reflexes in Ciona and other animals. The twitch resopnse gives an estimate of the minimal reaction time and shows that the long delay and variability in the timing of the general locomotory movements are due to central processes which determine the response to light in accordance with the general condition and activity of the animal at the time. Ichthyophis is sensitive to light falling anywhere on the head, and on the tail from above. The rest of the body is only slightly sensitive. Local illunination on the head or tail gives rise usually to the general locomotory response, but with caudal illumination a local reflex with a very short reaction time precedes the general response. Judged both from reaction times and from numbers of responses obtained, the tail seems to be more sensitive to light than the head. Operations showed that for a general response to develop from caudal illumination, the spinal cord must be intact, but the local tail reflex is not affected by spinal section. The response of the head to light was not affected by severing the optic nerves. The time relations, sensitivity, general character and spatial distribution of the dermal light sense of Ichthyophis are discussed in relation to the mode of life of the animal and it is concluded that in all respects, its properties coincide very remarkably with the animal's needs.