630 HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY 



RANZANIDE5 



SYLLIS 



NEREIS 



PHYLLODOCE 



HAL/OTIS 



PATELLA 



SCUTUM 



SECRETION 



MUREX 



LIMULUS 



SCOLOPENDRA 



DYTISCUS LARVA 



— CUTICLE 



\ 



SAGITTA 

 AND 

 TRANSVER 

 SECTIONS 



FIG. 5. A comparative series showing degrees of development in ocelli of polychacte annelids 

 Qipper roiv), gastropod moUusks (^center row) and arthropods Qower row), the ocelli in longitudinal 

 section in all instances. No phylogenetic interpretation seems indicated. [Limulus after Demoll, 

 Scolopendra after Heymons, Dyliscus after Giinther, others after Hesse; from Milne & Milne (193).] 



ing, and extended over the soft ventral surface of the 

 foot. Moreover, as the animal aged, its response to 

 light changed from negative to positive. 



Among the pelecypods, Nagel (201) distinguished 

 a category of 'ikonoptic' organisms in which the 

 structure of the ocelli seemed suitable for producing 

 a poor image in the receptor cells. Potamides has a 

 single layer in the retina, Pecten (fig. 7, left) a double 

 layer; in both instances the ends of the receptor cells 

 are turned away from the lens so that the retina is 

 inverted (159, 209). Wenrich (291) investigated what 

 he believed to be image-formation in Pecten in terms 

 of the smallest white card intensely illuminated, 

 movement of which would produce a shell-closing 

 response in the scallop. A more probable explanation 

 for his observed fact is that, at the light intensity 

 used, the appearance or disappearance of the card in 

 the visual field furnished the minimum effective change 

 in brightness. Hartline (88) found by electrical means 



that the distal (smaller) layer of retina mediates a 

 strong off-response, whereas the proximal layer dis- 

 charges nerve impulses whenever illuminated. 



A remarkably gradual series can be arranged show- 

 ing sectional views of gastropod eyes (fig. 5, second 

 line), but the significance of the differences noted in 

 terms of photic responses by the intact animal may 

 be questioned (121, 216, 295). The sign of the re- 

 sponse appears to be altered by many other factors, 

 such as diet, wetness or dryness of the body surface 

 (196), and whether the animal is inverted or upright 

 (74). The role of general photosensitivity in these 

 reactions has not been segregated from the supposed 

 dependence upon vision through the ocelli. 



On the basis of embryonic origins and neurologic 

 connections, Hanstrom (82) classified ocelli in arthro- 

 pods into three categories: a) the nauplian eyes of 

 crustaceans, the ocelli of insects, the median eyes of 

 trilobites, the ocelli of xiphosurans and the eyes of 



