546 



THE BIOLOGY OF MARINE ANIMALS 



of the light-cell. Triopa fulgwans, Kaioplocamus ramosum and Plocamo- 

 phorus ocellatus are other luminescent nudibranchs. 



A myopsid squid Heteroteuthis dispar from deep water in the Mediter- 

 ranean emits a luminous cloud when disturbed, comparable to the dis- 

 charge of ink by shallow-water species. Although this has been considered 

 another instance of symbiotic bacteria, it is probable that the luminescent 

 secretion is produced by the animal. The luminous organ is a rather large 

 gland partially surrounded by the ink sac. It possesses a reservoir and 

 opens into the mantle cavity by two apertures. The gland is lined with low 

 epithelial cells, and is provided with muscles for squeezing out the secretion. 



Photocytes^ Nerve 



Fig. 13.7. Light-producing Glandular Cells of the 

 Gastropod Phyllirrhoe bucephala 

 A nerve is shown terminating on the photocyte to the right. (After Dahlgren, 1916.) 



A similar light gland is present in the Japanese squid Sepiolina 

 nipponensis {26a). 



Among the lower chordates various balanoglossids emit a luminous slime 

 from the whole surface of the body. Light-producing species include 

 Balanoglossus minutus and Ptychodera bahamensis. Extracellular luminesc- 

 ence has also been described in fish, but some of the species which dis- 

 charge a luminous secretion appear to harbour symbiotic bacteria, e.g. 

 Malacocephalus. Beebe and Crane (3) noticed a mucous luminescent coat- 

 ing adhering to the teeth of the bathypelagic angler-fish Linophryne arcturi, 

 and the deep-water eel Saccopharynx harrisoni has a pair of troughs along 

 its back which are filled with a bluish-white luminous substance. Many of 

 the deep-water anglers (Pediculati) are provided with a luminescent lure 

 (illicium) representing a modified first dorsal fin ray (Fig. 13.8). The escal 

 light-organ at the tip of the illicium has a transparent window through 

 which the light can shine and contains a glandular chamber lined with 

 luminescent cells. The mechanism of light-production in these fishes awaits 

 clarification (4, 28). 



