032 RADIATION BIOLOGY 



CHAETOGNATHA 



Sagitta and its relatives have two ocelli located dorsolaterally on the 

 head. According to Hilton (1921b), each is globular, with three bicon- 

 vex lenses separated by pigment; the lenses concentrate light on groups 

 of rodlike sensory cells. Russell (1931) looked into variations in the 

 vertical distribution of Sagitta in relation to light at various depths of 

 the sea but reached no specific conclusions regarding the photoreceptors. 



ANNELIDA 



Hilton again (1924) has provided the chief summary of anatomical 

 information regarding the eyes of archiannelids. In Dinophilus the eyes 

 consist of two semicircular pigment spots in the head region; in Nerilla 

 there are four eyespots, the anterior pair more elongate and directed out- 

 ward and forward, the posterior pair pointing outward and backward. 

 Nothing is known of their physiology. 



Polychaeta. The polychaetes, however, afford far greater range in 

 photoreceptors. Both Schreiner (1898) and Hesse (1899) attempted to 

 trace a phylogenetic sequence through progressively more complex forms, 

 from epithelial eyespots without separate lenses, through ocelli with 

 lenses as in Nereis, to the highly complex, camera-like eyes of Alciopa 

 and Ewpolyodontes. Unfortunately the anatomical information is not 

 matched by studies of the response characteristics of the various mecha- 

 nisms, so that it would appear that many with highly specialized eyes 

 achieve no more complex shadow reactions or true vision than those 

 depending on dermal photosensitivity or simple pigmented eyespots. 

 Thus in Potamilla, where there are numerous modified epithehal cells in 

 patches on the main stems of the cephahc branchiae, each is composed of 

 elongated pigment-bearing cells, a few of which possess in their distal ends 

 refractive bodies that seem to concentrate the light internally on a sup- 

 posedly photosensory portion of each cell. Every sensory element is 

 separated from its neighbors by pigment cells, making it a true com- 

 pound eyespot. Similar structures are found in many sabellids and 

 serpulids (such as Branchiomma, Fig. 14-5), yet the known reactions of 

 these forms seem not to differ from others having far simpler morpho- 

 logical details. 



The ocelli of Nereis (Fig. 14-6), Potamilla, and the like lie under len- 

 ticular thickenings of the cuticle, but the eyes themselves are composed 

 of a single layer of epithelium. A series can be arranged showing pro- 

 gressive enlargement and infolding of the cuticular lens until it forms a 

 conspicuous spheroidal body around which the sensory cells are arranged 

 almost radially, and the whole is enclosed by small pigment cells except 

 for an outer zone through which light enters (Fig. 14-6). In Branchi- 

 omma and some others the sensory layer around the cuticular lens is 

 double, with the pigmentless sensory cells separated from the lens by 



