624 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



In Peranema the whole cell, including its flagellum, 

 appears to he sensitive to light (238, 239). A gradient 

 can be detected from a minimum in the posterior end 

 to a maximum in the flagellum. Even detached flagella 

 will respond to increased illumination by bending, 

 but no recovery seems possible. Hence both the re- 

 ceptor and effector substances must be widely dis- 

 tributed in the protoplasm, but the recovery phase 

 depends upon transport of additional materials into 

 the flagellum from the cell body where they are 

 elaborated. 



Cells With Obvious Pholoreceplors 



Definite organelles called stigmata are present in 

 many flagellates and seem associated with a localized 

 photosensitivity. Some stigmata are ball-like masses 

 of opaque red or black pigment. This is the case in 

 Euglena where the stigma is close to the double base 

 of a single flagellum and must shade the flagellar 

 bases from radiations reaching the cell from straight 

 ahead. If it is assumed that shading allows the flagel- 

 lar mechanisms to operate at full speed and illumina- 

 tion from the side inhibits the lashing movement, then 

 the polarity of swimming movements with respect to 

 a point source can be explained rather simply. 



Cup-shaped and spoon-shaped stigmata are usual 

 among colonial flagellates such as Gonium and Volvox. 

 The concavity of the stigma is associated with the 

 hypersensitive protoplasm and may be lined with a 

 reflecting layer which serves as a concave mirror and 

 concentrates the light at a focal point with the photo- 

 sensitive region. Both Gonium and Volvox stigmata 

 possess a lens as well. In Volvox the size of the stig- 

 mata decreases with distance of the cell from the an- 

 terior pole of the colony, and all stigmata are placed 

 so as to face outward and slightly toward the an- 

 terior pole. The two flagella of each cell beat in dif- 

 ferent modes and at unlike rates according to the 

 direction from which light reaches the stigma. Mast 

 worked out the paths of the reflected and refracted 

 rays (180, 182) but did not identify the functional 

 connection between the photosensitive mass in the 

 stigmatic area and the locomotor mechanism at the 

 flagellar bases. It is clear, however, that when a Volvox 

 colony is illuminated only from directly ahead on its 

 axis of symmetry, every cell receiving radiations does 

 so in ways which lead to symmetrical beating of the 

 flagella. Under these circumstances the entire colony 

 revolves on its axis and, unless the light intensity is 

 excessive, approaches the source while so rotating. 

 Unilateral illumination, by contrast, appears to 



modify flagellar movements on the illuminated side 

 while vigorous beating on the shaded side gradually 

 turns the colony until its axis is directed toward the 

 source. 



Mast (182) presented generalizations concerning 

 the form and function of stigmata in unicellular and 

 colonial flagellates, without mentioning the most re- 

 markable of them all. In Pouchetia and related dino- 

 flagellates, the lens associated with the stigma is 

 enormous and spherical. The resemblance to a multi- 

 cellular eye in these unicellular organisms is striking. 

 No experimental work has been reported which 

 might show the use to which Pouchetia puts this 

 striking organelle. 



PHOTOSENSITIVITY IN MULTICELLUL.'^R ORG.ANISMS 



With multicellularity a metazoan might be ex- 

 pected to show pronounced localization of photo- 

 sensitivity into obvious eyes. Some metazoans manage 

 quite well and respond to light without obvious 

 specializations of this kind. Others, although equipped 

 with eyes, seem to ignore visual cues for considerable 

 parts of their life histories. 



Photosensitivity Mediated Without Obvious Receptors 



To this phenomenon the phrases 'dermoptic sense' 

 and "dermal photosensitivity' have often been ap- 

 plied (e.g. 133-135, 201). Table i indicates the 

 taxonomic groups in which a generalized response of 

 this kind has been demonstrated. Often a failure to 

 recognize the presence of this photosensory system in 

 animals with eyes has led to wrong conclusions con- 

 cerning the effects of unilateral blinding. 



Ganglionic Photosensitivity 



In 1934, Welsh (290) and Prosser (218) discovered 

 independently that the abdominal ganglia of the 

 crayfish were photosensitive, permitting the animal 

 to respond to light even after its eyes had beeti re- 

 moved. Hess (113) found the same sensitivity in 

 abdominal ganglia of the shrimp Crangon and the 

 spiny lobster Panulirus but learned that photosensory 

 cells were scattered along nerves in such remote 

 parts of the body as the uropods. The role of gangli- 

 onic photosensitivity in controlling locomotor ac- 

 tivity of the intact animal has received some con- 

 sideration (232). Probably it is more important at 

 the time of molt, before the new exoskeleton has de- 



