LUMINESCENCE 



549 



Polynoid worms such as Polynoe and Acholoe show transitory flashes of 

 light in their elytra when stimulated. The light arises in a single layer of 

 photogenic cells lying on the lower surface of the scale and passes through 

 the latter before reaching the exterior. The photogenic cells are provided 

 with a rich innervation which originates in a central elytral ganglion 

 (Fig. 13.10) (5, 45). 



Intracellular luminescence is widespread among euphausiids and shrimps, 

 and in some species the photophores are highly differentiated organs, pos- 

 sessing reflecting layer, screen and lens, associated with a group of photo- 

 genic cells. Among the euphausiids there are three to ten photophores 

 arranged on the eye-stalks, thorax and abdomen. In their simpler form they 



Elytral nerve 



Epidermal cell 



Fig. 13.10. Section through Elytrum of Acholoe astericola, 

 Showing Photocytes and Nerve Supply 



are cup-shaped and the photogenic cells are continued distally into a rod- 

 shaped mass. Bounding the cup externally is a refractive lamellar body. 

 A stout nerve penetrates each photophore, and subdivides and ramifies 

 among the photogenic cells. More complex photophores, such as those of 

 Meganyctiphanes norvegica, contain lenses and a thickened corneal layer 

 in addition (Fig. 13.11). Trojan, who has studied the optical properties of 

 the photophores in Nyctiphanes couchi, has shown that the light-rays are 

 brought into focus in front of the light-organ by a refractor and biconvex 

 lens. 



The photophores of pelagic shrimps (Caridea and Penaeidea) are distri- 

 buted over the appendages and the thoracic and abdominal sterna in such 

 a way that the light they emit is directed downwards. Their organization 

 is rather simple in Systellaspis (=Acanthephyra) debilis, in which some of 

 the photophores are narrow elongated structures having a cylindrical lens 

 formed by a thickening of the cuticle, and an underlying layer of photo- 

 genic cells. Other organs are spherical and are bounded externally by an 



