MITES 233 



parts of the globe, with large areas in between where they are altogether 

 absent. These two facts suggest that the Pentastomida is a group of 

 great antiquity, a supposition which is supported by the morphology 

 of the tongue-worms themselves. The single species which in its adult 

 stage parasitises birds, Reighardia sternae (order Cephalobaenida), is 

 placed in a genus and family of its own. It has not yet been recorded 

 from Britain but as three out of the five known hosts are on the British 

 list it seems highly probable that in due course it will be found here. The 

 species has been collected from the common tern in Italy, North 

 America and China, from the ivory gull in Greenland, and the glaucous 

 gull from Lapland. The bird tongue-worm is therefore widely distri- 

 buted, but in this case the discontinuity may be more apparent than 

 real, as collectors of pentastomes are rather rare and widely distributed 

 themselves. The intermediate host is not known, but a fish of the herring 

 group seems a reasonable guess. On the other hand nymphal stages 

 have been found in the veins of the common tern, which suggests that 

 development could be direct, without the help of an intermediate host. 



There is a single record of a larval form found in a bird on the 

 British list, namely Armillifer armillatus from the honey-buzzard in 

 Sweden. This species is a parasite of pythons in the adult stage, but like 

 many of the spiny-headed worms it apparently has the power of en- 

 capsuling in a variety of hosts if ingested accidentally and has also 

 been found at this stage in man, leopards and dogs ! 



The great interest of the tongue- worms lies in the fact that they are so 

 modified, apparently by their parasitic mode of life, that no agreement 

 has been reached with regard to their correct place in the animal king- 

 dom. At one time or another they have been classified with the flat- 

 worms, and the roundworms — in fact with all the major groups of 

 parasitic worms. To-day (1946) they are placed with the Arachnida in 

 the Zoological Record. Chandler (1946), however, considers them to be 

 a separate and aberrant class of arthropods. Baer (1946) states that the 

 tongue-worms are now attached to the annelids or segmented worms. 

 Heymons, who is probably the greatest authority on the group, 

 cautiously suggests that they occupy a position somewhere between the 

 arthropods and the annelids. Their life-cycle, which involves an inter- 

 mediate host, is certainly reminiscent of the parasitic worms. The first 

 larval stage when it emerges from the tggy on the other hand, has two 

 pairs of vestigial legs and resembles a mite. Some authors regard these 

 legs as atrophied appendages of the arthropod pattern, while others 



