DIFFICULTY OF PROBLEM. 



189 



Milne Edwards, and subsequently by other authors, 

 especially by Leydig.* The short stalk cootains 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 antennee of 

 Pleuromma,t the appendages 

 on the second thoracic leg of 

 Serolis, those on the maxilli- 

 peds of Enrycopa, on the me- 

 tatarsus of spiders, the finger- 

 shaped organ on the antennas 

 of Polydesmus, the singular 

 pleural eye (?) of Pleuromm^), 

 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 microscdpes, the use of 

 new reagents, and of mechanical appliances, such as 

 the microtome ; but the ultimate atoms of which matter 

 is composed are so infinitesimally minute, that it is 

 difficult to foresee any manner in which we may hope 

 for a final solution of these problems. 



Loschrai'lt, who has since been confirmed by Stoney 

 and Sir W. Thomson, calculates that each of the 



* Zeit. fur Wiss. Zool," 1878. 



t Brady, " On the Copepoda of the Challenger Expedition," vol. viii. 



Fig. H7. — Senee-orRan of leech 

 (from Carriere, after Ranke). 

 1, Epithelium ; 2, pigment ; 3, 

 cells ; 4, nerve. The longer axis 

 equals '4. mm. 



