Respiration in Invertebrate Animals. 149 



deposited in lines, as the cilia are disposed along the branoiual 

 leaves of the Lamellibranchiate Mollusks, leaving a non-ciliated 

 interval to which the active process of respiration is chieHy 

 limited. The gases must consequently traverse the entire sub- 

 stance of the calcareous and membranous coats. The inter- 

 stices between the calcareous particles might on this view be 

 considered as each representing a cell, in which a small volume 

 of air is held stationarily in intimate contact with the blood, 

 and beyond the disturbing control of the ever-moving and vary- 

 ing parietes of the general cavity ; and in which, as in the air- 

 cells of Manunals, the interchange of the gases is a continuous, 

 not an interrupted, process. But these permeable calcareous 

 coats, while they divide the oxygen into myriads of infinitesimal 

 portions, bringing it thus in a state of extreme subdivision into 

 contact with the blood, act also like other porous bodies upon 

 gases, by condensing their volumes. The power thus exerted 

 increases the diffusiveness of the gases, and consequently a\ig- 

 ments the measure in a given time of the function of the part, 

 because it virtually accelerates the interchange of the gases. 

 The ultimate vessels of the abdominal organs are furnished with 

 soft non-calcareous coats. The lime in these parts of the body 

 is present only on the larger trunks. This substance is sup- 

 pressed, therefore, in those organs in which its presence would 

 interfere with the nutritive and secernent office of the minute 

 vessels. Being present on those of the lung, the inference is 

 unavoidable, that in this situation at least it does not obstruct the 

 function of the organ. It seems on the clearest grounds that the 

 mechanical subdivision of the air in a respiratory organ may be 

 made to supersede the necessity for the subdivision of the blood 

 by the formation of a rete mirabile. To this end in the Lima- 

 cidaj a contrivance of singular simplicity is adopted : the air 

 is made to rush in steady but infinitely divided currents in tlie 

 direction of the blood. Tliis is enough to secure the intended 

 result. The blood accordingly flows in channels of compara- 

 tively large diameters. Coarse trunks separated by wide inter- 

 vals, they contrast most strikingly with the elaborately formed 

 parallel cajiillaries of the gills of all the branchiferous orders of 

 Gasteropods, in which the blood-stream is reduced to the utmost 

 minuteness. 



On no other interpretation of the anatomical facts by which 

 the pulmonary sac of the air-breathing Gasteropods is distin- 

 guished, is the reproach of rudeness and coarseness of con- 

 struction to be removed. On this interpretation the rudeness 

 is turned into subtlety and the coarseness into refinement, and 

 the physiologist may cue indeed an organ which at first only 

 shocked the short-seeing mind with a sense of disappointment, 



