334 THE BIOLOGY OF MARINE ANIMALS 



reception. When an ommatidium is stimulated with brief flashes of light 

 a train of impulses appears in the optic nerve, the number of which depends 

 on the duration and intensity of the stimulus (Fig. 8.21). By varying these 

 two factors it has been possible to determine the flash duration which just 

 produces a single impulse at various intensities. The data so derived show 

 that the reciprocity law (duration X time = a constant) holds for the 

 production of a single impulse. This reciprocal relationship, found in the 

 responses of photoreceptors, has been attributed to photochemical pro- 

 cesses in the eye, and may be an expression of the Bunsen-Roscoe law of 

 photochemistry. 



Recovery of sensitivity during dark-adaptation in single ommatidia of 

 Limulus is illustrated in Fig. 8.22. The eye was stimulated by brief flashes 

 of light at various intervals after placing the preparation in the dark, and 

 spike potentials were recorded from a single optic-nerve fibre. The number 

 of spike potentials is low at first, increases rapidly during the initial period 

 of dark-adaptation and more slowly thereafter. The time course of re- 

 covery is followed by determining the intensity of a flash necessary to 

 elicit a single impulse, or by measuring the number and frequency of 

 impulses evoked by a constant flash at selected intervals. The rate of 

 recovery depends on the previous light history of the eye, varying with the 

 intensity and duration of previous light adaptation (62). 



Spectral-sensitivity curves for Limulus and Eledone derived from measure- 

 ments of retinal action potentials and optic nerve potentials are shown in 

 Fig. 8.18. The action spectra obtained by such means generally agree with 

 spectral-sensitivity curves obtained from behaviour studies. There is 

 reasonably close resemblance between spectral-sensitivity curves and the 

 absorption curves of visual pigments (rhodopsin) in those instances where 

 data from both sources are available (63). 



Many arthropods are sensitive to polarized light and some species make 

 use of this ability in orientation. Cladocerans and amphipods orientate to 

 polarized sky light, and mysids and hermit crabs show distinct reactions 

 to plane-polarized light vibrating in different directions. The compound 

 eye of arthropods acts as a polarization analyser. When a single ommatid- 

 ium in the eye of Limulus is stimulated by short flashes of plane-polarized 

 light and spike potentials are recorded from the optic nerve, the discharge- 

 rate is found to vary with the plane of polarization. Stimulation is maximal 

 near 0° and 180° with respect to some particular setting, and is minimal 

 at 90° where only half as many impulses result from a standard flash 

 (7b, 149, 165). 



MECHANORECEPTION 



Tactile stimulation produces mechanical deformation of cellular surfaces. 

 The corresponding sensitivity may take the form of a generalized cellular 

 irritability, as in the independent effectors of sponges, or involve the stimu- 

 lation of free nerve endings or specialized tactile receptors. More sensitive 

 mechanoreceptors detect pressure waves created by vibration of distant 



