INVERTEBRATE PHOTORECEPTORS 



635 



tion of the ocelli gave the same unilateral blinding effect. Hess (1931) 

 concliuled that the reactions to light intensity were mediated through 

 dermal photoreceptors along the middorsal line, sides, and anterior and 

 posterior ends of Eunice, but that the ocelli served visual functions apart 

 from those ehciting photokinetic responses. Other anatomical studies 

 on polychaetes involve the compound eyespots of Spinther and the recep- 

 tors of Polygordius, Tomopteris, and Scolccolepis. Behavior studies in 



ANNELID 

 (ALCIOPA) 



GASTROPOD 

 (PTEROTRACHEA J 



CRUSTACEAN 

 (COPILIA) 



VENTRAL VIEW 



SECRETION. ACCOMMODATION 



ACCOMMO- 

 DATION 

 MUSCLE 



OPTIC 

 NERVE 



Coptic 



NERVE 



Fig. 14-7. The camera-style eye is represented not only in the cephalopod mollusks 

 but also among the annelid worms and, with specializations, among the gastropod 

 molliisks and plankton crustaceans. The Alciopa eye appears to provide a good image 

 of fair size, but the sensory mechanisms in Pterotrachea and Copilia must function 

 more as sights for aligning the body with reference to light. All three provide for 

 accommodation. (Annelid Alciopa after Demoll, 1909b; gastropod Pterotrachea after 

 Hesse, 1910; crustacean Copilia in dorsal vieiv after Giesbrecht, 1890; detail after 

 Grenacher, 1879.) 



terms of photoreception include work on the serpulid Hydroides, on 

 Sabellaria larvae, and Clitellio. 



Oligochaeta. Possibly the wider availability of earthworms has made 

 easier the study of this more degenerate type of organization. Neuronal 

 photoreceptors in the skin have been recognized and described, by Lang- 

 don (1895), Hesse (1896), and Hess (1925). Reactions to light in these 

 animals (Allolobophora, Eisenia, Lumhricus, Perichaeta, and Pheretima) 

 have been analyzed. Extension of the body exposes more photoreceptors 

 and hence increases the worm's sensitivity to light (Harper, 1905) ; in 



