MICRO-PARASITES 239 



eurella aviseptica), anthrax [Bacillus anthracis), vibrio infection {Vibrio 

 metchnikovi) , infectious coryza [Haemophilus gallinarum) and various otlier 

 diseases caused by the Streptococcus and Staphylococcus type of bacteria, 

 apart from those ah'eady mentioned. Nevertheless the bacterial dis- 

 eases of wild birds, particularly in Britain, are virtually unknown. 



The organisms described above are all parasites with a marked 

 pathological effect on the host. There are, however, numerous bacteria 

 which constitute the normal flora of an animal's body. Some of these 

 are harmless commensals, others are parasites which have no noticeable 

 effect except in certain special circumstances when they become 

 dangerous, and some are definitely beneficial to the host and are thus 

 more correctly labelled symbionts. There is considerable evidence, for 

 example, that certain bacteria in the intestines of mammals and birds 

 synthesise proteins, vitamins and possibly essential amino-acids which 

 are then used by the host. Some of the invertebrate parasites of birds 

 apparently depend on the presence of the bacteria associated with the 

 alimentary canal, for they cannot survive without them. It is always 

 more difficult to study the beneficial rather than the harmful bacteria, 

 for the effects of the former are so much less dramatic. In a few cases, 

 such as the deep sea fish with luminous organs, the relationship is fairly 

 obvious. The fish possess definite hollow structures, generally situated 

 on some part of the head, supplied with specialised secretory glands. 

 The bacteria are present in the water and filter into these special 

 organs from outside. Once they have gained access they find them- 

 selves in the presence of a perfect nutrient medium secreted by the 

 glands in which they luxuriate and multiply rapidly. The highly phos- 

 phorescent areas which result from these dense aggregations of luminous 

 bacteria serve the fish as recognition marks, warning signals, or lures to 

 attract their prey. Very few symbiotic relationships between bacteria 

 and vertebrates present such a clear cut picture as this, but it is never- 

 theless highly probable that they do in fact exist between birds and the 

 microscopical organisms which live in their bodies. This case must not 

 be confused with the occasional records of luminous bacteria on the 

 plumage of owls which cause the birds to glow faintly and have given 

 rise to many terrifying ghost stories. This luminescence is due to the 

 accidental presence of saprophytic forms which are commonly found 

 growing on dead meat and fish. The owls acquire them temporarily 

 from the carcase on which they have been feeding. 



