146 THE WONDER OF LIFE 



rivers of the Andes, where there is a rapid succession of 

 falls, cascades, and potholes. Under usual conditions 

 Arges is a clumsy and awkward swimraer, but for creeping 

 and climbing in the torrents it is wonderfully adapted. It 

 anchors itself by its suctorial mouth, and works itself up- 

 stream with the help of a ventral bony plate bearing the 

 ventral fins and equipped with strong muscles which move 

 it backwards and forwards. The plate is studded with 

 small sharp teeth pointing backwards. These catfishes 

 climb up the smooth water-worn surfaces of deep potholes, 

 and have been known to ascend eighteen feet without a slip 

 or fall. 



Terrestrial Animals Under Water. — On the Mediter- 

 ranean shore among the calcareous Algae, Racovitza found 

 a marine spider, which Louis Fage has described under the 

 title Desidiopsis racovitzai. It lives in crevices, in burrows 

 (of Lithodomus), in empty shells (of Vermetus), and keeps 

 the water out more or less by spinning threads across the 

 entrance to its retreat. There is no tide to contend with, 

 but it is a strange abode for a terrestrial animal. Unlike 

 the freshwater spider, it cannot swim. It can remain 

 for a long time under water, but has to return to dry 

 land periodically to get a supply of air, which is entangled 

 about the posterior body. What the creature feeds on is 

 uncertain. 



A species of mite, Erythrceus passerinii, belongirig to a 

 terrestrial stock, is known to Kve in the crevices of the sea- 

 shore rocks, and to be able to withstand prolonged immer- 

 sion. It utihzes the air imprisoned in the capillary passages 

 in the cracks of the rocks. A primitive wingless insect, 

 Anurida maritima, which has been carefully studied by 

 Imms, lives habitually among the sea-shore rocks. When 



