KNOWLEDGE OF THE HYDRACHNIDAE. 273 



altogether inconsistent with the essentially subaqueous habits 

 of the Hydrachnidae * and I have therefore been hitherto more 

 inclined to adopt the view that they absorbed air into their 

 trachea through the general surface of their integument, the 

 respiratory system being thus, as originally suggested by Duges, 

 most nearly comparable with the closed tracheal or tracheo- 

 ventral system of the Insecta. I was scarcely prepared, how- 

 ever, to find how close an analogy subsists between the tw^o groups 

 with respect to their air system as has now come to light. In 

 the first instance, wdth the view to arrive at a correct estimate 

 of the origin and distribution of the tracheal tubuli, sections 

 were made of fresh examples of various species, but more 

 particularly of Limnesia histrionica, some with the freezing 

 microtome and others with simply a fine pair of scissors. Their 

 bodies being laid open, it was found that the tracheae converging 

 from all regions formed a thick plexus upon the wall of the 

 stomach and oesophageal tract ; and no more extensive system 

 of tubuli apparently passing off thence to the anterior region 

 than was distributed to other portions of the body, such as 

 the locomotive limbs, it was anticipated as not improbable that 

 the thick tracheal plexus coating the stomach and oesophagus 

 extracted air from the water taken into the body through 

 the mouth and held in suspension by these viscera. It was 

 only by carefully detaching and examining separately the 

 rostra, bearing the mandibular and palpiform appendages, from 

 a considerable series of examples that the true condition of 

 affairs was revealed. The two tracheal tracts described by 

 Kramer as leading to the summit of the rostrum were now dis- 

 tinctly shown ; but in place of terminating there as stigmatal 

 openings as described by him and Dr. Haller, it w^as found that 

 from each of these points was dev^eloped an exceedingly delicate 

 film-like saccular expansion of the surface of the integument into 

 which the tracheal tubuli ascended and formed a convoluted plexus, 

 in a manner altogether identical wdth what obtains in the leaf- 

 like tracheal -gills of the larvae of the Ephemeridae and other 

 aquatic insects. The two structures are in point of fact pre- 

 cisely homologous, and it is an interesting circumstance thus to 



* Many species are now known to be limited in their distribution to the 

 deeper waters of the Swiss lakes, while specimens kept in aquaria live 

 for months without coming to the surface of the water. 



