136 On the Meclianism of Aquatic Respiration. 



fact explains an observation which the author has often made, 

 that, considering the exalted muscular activity of the Articulata, 

 of insects in particular, the current of the blood in its channels 

 moves at a disproportionately slow rate. This diminished velocity 

 is compensated, in the most perfect manner, by the rythmic con- 

 tractility possessed by the parietes of the tracheae. In virtue of 

 this property the required motion is imparted to the air rather 

 than to the blood. As well observed by M. E. Blanchard, the 

 voluntary muscles of the body by their peculiar distribution 

 favour the course taken by the blood, both in the arterial and 

 venous moiety of the system. No valves exist. The mechanism 

 of nutrition and respiration in the tracheary Articulata would be 

 most unsuccessfully studied without the guiding knowledge of 

 these general facts. 



The characters then which distinguish an annulose from an 

 articulate animal are more deeply graven in the interior of the 

 organism and in its physiological actions than on the visible and 

 exterior forms by which they are respectively characterized. 

 There is deep meaning in the freaks of nature. 



In the water -breathing Articulata the system of air-tubes just 

 described suddenly disappears. The large size of the crustacean 

 renders practicable the introduction of complex branchial organs. 

 They accomplish that office which in the myriapod and the in- 

 sect devolved upon the aeriferous tracheae. Nature has nowhere 

 blended the two methods of respiration in the same class. There 

 exists no adult water-hreath'mg myriapod or insect. An ai?-- 

 breathing crustacean can nowhere be found. Her plans are con- 

 sistent. 



One more general fact of organization with respect to the 

 Articulata remains to be stated. Nowhere from the myriapod 

 to the crustacean is any trace whatever to be discovered of the 

 existence of a true ciliary epithelium ! Why should a structure so 

 constant and profuse in all classes below the Articulata so sud- 

 denly and so completely disappear at the lower limit of this 

 class ? If in the epidermic system of the articulated animal 

 there be something incompatible with the evolution of the 

 ciliary variety of epithelium, why should it not occur on the 

 mucous ? In no single instance, in any species of myriapod, in- 

 sect or crustacean, on the mucous tract of the alimentary canal, 

 or anywhere else, has any indication whatever of the presence of 

 vibratile epithelium been ever yet discovered. This extraordinary 

 fact cannot be arbitrary and unmeaning. Ciha are here sup- 

 pressed for some reason and from some cause — what can it be ? 

 The solution should be sought in the rapid and unwonted evo- 

 lution of the muscular system which takes place at this point. 

 In the organism of the articulated animal there does next exist 



