DIFFICULTY OF PROBLEM. 



189 



Milne Edwards, and subsequently by other authors, 

 especially by Leydig.* The short stalk contains a 

 canal, which appears to divide into radiating branches 

 on reaching the " slipper," 

 which itself is marked by a 

 series of rings. 



Among other problematical 

 organs, I might refer to the 

 remarkable pyriform sensory 

 organs on the antennae of 

 Pleuromma,t the appendages 

 on the second thoracic leg of 

 Serolis, those on the maxilli- 

 peds of Eurycopa, on the me- 

 tatarsus of spiders, the finger- 

 shaped organ on the antennae 

 of Polydesmus, the singular 

 pleural eye (?) of Pleuromma, 

 and many others. 



There is every reason to 

 hope that future studies will 

 throw much light on these in- 

 teresting structures. We may, no doubt, expect much 

 from the improvement in our microscopes, the use of 

 new reagents, and of mechanical appliances, such as 

 the microtome ; but the ultimate atoms of which matter 

 is composed are so infinitesiraally minute, that it is 

 difficult to foresee any manner in which we may hope 

 for a final solution of these problems. 



Loschmidt, who has since been confirmed by Stoney 

 and Sir W. Thomson, calculates that each of the 



Fi,?. 117. — Sense-organ of leech 

 (from Carriere, after Ranke). 

 1, Epithelium ; 2, pigment ; 3, 

 cells; 4, nerve. The longer axis 

 equals ••! mm. 



* Zeit. fur TFtss. Zool.,'' 1878. 



t Brady, " On the Copepo^la of the Challenger Expedition," vol. viii. 



