CH. I] LAND PLANARIANS. 53 



resemblance between the South African genus Opisthocen- 

 trus and the Panaman Opisthacantkus, and between the 

 only two genera of the Diplocentridae, Diplocentrus and 

 the Arabian Nebo. This may be merely a relic of former 

 warm periods prevailing in the north, of which the 

 existence of Tityus in amber of the Baltic is a further 

 indication. This genus now occurs south of the tropic of 

 Capricorn, thus indicating a state of climate favourable to 

 the migrations of scorpions by Behring's Straits. 



The Distribution of Land Planarians. 



This group of worms is one that should be of great 

 use to the student of geographical distribution. The land 

 Planarians are of course of an ancient stock though 

 the modern representatives may be recently derived from 

 some one branch of this stock. They are purely terres- 

 trial animals, always an advantage in considering the 

 problems of Zoogeography ; and finally it is probable, 

 though it is uncertain whether there are any actual 

 facts that can be alleged in support of the contention, 

 that the animals, having a coating of cilia and secreting 

 from their skin a slimy mucus, would be destroyed by 

 contact with salt water. It is however only recently that 

 attention has been actively directed towards the study of 

 this group of the Planarian worms. The late Prof. 

 Moseley took advantage of the opportunities afforded him 

 during the cruise of the " Challenger " to collect and 

 describe a considerable number of new forms ; the litera- 

 ture which he gives of the subject in his paper 1 shows 



1 Quart. Journ. 3Iicr. Set. Vol. xvu. 



