308 THE BIOLOGY OF MARINE ANIMALS 



of echinoderms under varying conditions of illumination it has been found 

 that the entire surface of the animal is light-sensitive. Similarly, when the 

 pigmented eye-spots or eyes of flatworms and polychaetes are removed, 

 there is still a residual and generalized light-sensitivity. In these and other 

 instances photoreception may involve stimulation of general unicellular 

 sensory elements distributed over the whole body surface. Alternatively, 

 the nervous system itself may be photosensitive and act as a primary 

 receptor. Simple unicellular photoreceptors are found 

 in the general epidermis of annelids, siphons of lamel- 

 libranchs, etc. (Fig. 8.3) (7, 95, 107, 108). 



Stigmata and Ocelli. Stigmata and ocelli are small 

 cups containing a light-sensitive surface backed by 

 pigment, and often enclosing a lens-like body. Simple 

 structures of this kind are found in certain flagellates 

 (52, 105). Multicellular ocelli range in complexity from 

 a simple sensory surface with intermingled sensory 

 and pigmented cells to cup-shaped organs provided 

 Fig. 8.3. Optic with cornea and lens. Among medusae pigmented eye- 

 Organelle (uni- S p t s are often present at the bases of the tentacles. In 

 cellular photo- Sarsia f or exa mple, the photosensitive surface forms a 



RECEPTOR) IN THE . u v i i-i u A tu 



Siphon of Mv CU P contaimn & a hyaline lens-like body. The receptor 

 arenaria (from e l ements in this ocellus are bipolar primary neurones 

 Light (95).) separated from one another by pigmented supporting 



cells. 



Similar cup-shaped ocelli occur in members of many other phyla, e.g. 

 flatworms, nemertines, larval and adult polychaetes, molluscs, chaeto- 

 gnaths and tunicates. In some polychaetes the eye-spots contain a lens 

 which concentrates light on the retinal surface (Fig. 8.4). A wide variety 

 of ocelli is found in gastropods and lamellibranchs. The eye-spot of 

 Patella is a simple sensory pit lying at the base of the tentacle; in Murex 

 the eye is cut off from the epidermis and encloses a large spherical lens 

 (Fig. 8.5). The scallop (Pecten) possesses numerous pallial eyes, each of 

 which is a small sphere with cornea, lens, retina and chorioid backing. The 

 retina consists of two layers, one behind the other: the two layers are 

 supplied by separate rami of the optic nerve and are covered by ganglion 

 cells. 



The median eye of crustaceans is essentially a group of three ocelli, one 

 central and two lateral. Each ocellus is a pigmented cup containing a lens 

 and lined with retinal cells from which nerve fibres proceed to the brain. 

 A median eye develops in the nauplius larva and is found in many adult 

 entomostracans. 



Limulus possesses two pairs of eyes, two lateral and two median. The 

 lateral eyes, known as facet eyes, are aggregates of ocelli. Each ocellus con- 

 tains a lens-like thickening of the cuticle, forming a conical projection ex- 

 tending into the interior of the eye. Beneath this is the photosensitive surface, 

 containing several kinds of cells. Among them are retinular cells arranged 



