CRUSTACEA OF THE DEEP SEA 125 



descends to depths of at least 2,000 fathoms ; and 

 he suggests that the Crab may be able to see its way 

 by the light emitted by the zoophytes. 



Some of the Crustacea, however, are themselves 

 luminous. Thus, Alcock records how specimens of a 

 deep-sea Prawn, Heterocarpus alphonsi, "poured out, 

 apparently from the orifices of the ' green glands ' at 

 the base of the antennae, copious clouds of a ghostly 

 blue light of sufficient intensity to illuminate a bucket 

 of sea-water so that all its contents were visible in 

 the clearest detail." Certain other Prawns are 

 known to possess special light-producing organs on 

 various parts of the body and limbs. It is in the 

 Euphausiacea, however, that these organs have been 

 most fully examined, and although the members of 

 this group (see Fig. 24, p. 56) are by no means all 

 deep-sea animals, some of them occurring at the 

 surface of the sea, the structure of their luminous 

 organs, or " photophores," may appropriately be 

 described here. They are situated on the under- 

 surface of the abdomen, in the basal segments of 

 some of the thoracic legs, and on the upper surface 

 of the eye-stalks. Each consists of a globular cap- 

 sule covered by a layer of pigment, except on the 

 outer side, where there is a transparent biconvex 

 lens. In the centre of the capsule is a peculiar 

 " striated body " which seems to be the actual seat 

 of luminescence, and behind it is a concave reflector 

 composed of concentric lamellae, and having a silvery 



