n6 HABITUDES AND ATTITUDES 



is bent down. The overlapping between head 

 and pronotum of parasites which live in the fur 

 of mammals renders the surface uniform and 

 more suitable for gliding through the fur, and 

 is a secondary development which has taken 

 place independently in these not nearly related 

 insects [Jordan, foe. citl\. 



The identification of the species of arthropods, 

 land-planarians, earthworms, and molluscs found 

 in or about a single fallen log is a matter of 

 almost unspeakable difficulty under the present 

 conditions of zoological nomenclature and publica- 

 tion. By far the greatest amount of biological 

 interest connected with a particular species may 

 be due to the association under which it was 

 found, but this is a matter of the least possible 

 systematic importance, except in the case of 

 parasites. The mere commingling of different 

 forms, apart from structural details, constitutes 

 a phase of our subject which is worthy of notice, 

 and may be termed bioccenotic convergence. 1 



It is known and will be found recorded in 

 Darwin's " Monograph of the Cirripedia," that 

 barnacles are frequently found attached by their 

 peduncles to the skin of sea-snakes (Hydrophidae). 

 Occasionally a considerable cluster of barnacles 

 may be fixed upon the flattened tail of a slender 

 snake, as in the case of a Plydrus platurus 



1 Such expressions as this are introduced merely for con- 

 venience of reference and classification of the phenomena, 



