PHYSIOLOGICAL 423 



luminous fungus. The shining of the cat's eyes in the dark (though 

 never in a really dark room) is merely an interesting reflection of 

 faint rays from the highly developed tapetum, and is comparable 

 to the shining of the luminous cave-moss (Schistostega), where 

 certain lens-like epidermic cells act as reflectors. In a simpler way, 

 a "luminous frog" had to be excluded from the list, when it was 

 found to have gorged itself with fire-flies. 



Although luminescence has been occasionally reported from 

 animals living in fresh water, e.g. in the larvae of a harlequin fly 

 (Chironomus), and in those of a fire- fly from Celebes, Harvey will 

 not admit that it occurs except on land or in the sea. Its well- 

 documented occurrence in a few freshwater animals may be due 

 to the ingestion or the adhesion of luminescent bacteria; yet this 

 criticism, as we shall see, has proved a sword that has pierced the 

 critic's own hand. For there are now many adherents to the theory 

 that a large number of luminescent animals are luminous simply 

 because they are habitually and heavily infected with luminous 

 bacteria. In this connection we should notice the curiously sporadic 

 occurrence of animal luminescence. For while there are classes, 

 like the Ctenophores, in which all the genera and species are 

 luminescent, it often happens that within a class only a few isolated 

 types have this peculiarity. Thus while the boring-shell Pholas is 

 one of the best-known cases of luminescence, it stands in this respect 

 very much alone among bivalve molluscs. 



Luminescence occurs in various Infusorians, like Noctiluca and 

 Pyrocystis, and in some Radiolarians ; in none of the Sponges; in 

 numerous Stinging Animals or Ccelentera, such as Sea-pens (Penna- 

 tulids) and the Portuguese- Man- of -War (Physalia); in all the 

 Ctenophores (which many zoologists would keep apart from the 

 Coelenterates) ; in sundry worm-types, including several earthworms 

 (Photodrilus), but in no leeches; in isolated star-fishes and in not 

 a few Brittle-stars or Ophiurids ; in no Lamp-shells (Brachiopods) ; 

 in no Moss Animals (Bryozoa or Polyzoa) ; in a multitude of Crusta- 

 ceans, large and small; in two or three Myriopods, e.g. Geophilus 

 electricus, one of the Centipedes; in numerous insects, especially 

 among Elaterid and Lampwid beetles; in no Arachnids (except 

 a dubious Indian spider); in a very few Bivalves, like Pholas; in 

 a few Gasteropods and Squids. In the large class of Tunicates 

 there are only a few instances of luminescence, one of them the 

 gorgeous free-swimming Pyrosome, which may be a yard in length 

 and give light enough to allow the observer to read a printed page. 

 There are many instances among marine fishes, especially among 

 those that frequent considerable or great depths. As to plants, 

 bio-luminescence is confined to Bacteria and Fungi; for the oft- 

 quoted story of Linnaus' daughter seeing light from dittany flowers 

 (Dictamnus) is not verifiable. 



