Respiration in Invertebrate Animals. 417 



they are much less visible than they are in the latter case, but 

 in anatomical characters and relations they are in both cases 

 most remarkably analogous. Now the interval between the two 

 layers which is crossed by the threads just described, is indubi- 

 tably filled with some fluid. What that fluid is, it is impossible 

 at present to say. 



The water-currents excited by cilia on the flat surfaces of the 

 branchial laminae set, from the fixed, in the direction of the free 

 border. The cilia which are distributed over the flat faces of 

 the leaflets are very much smaller than those which fringe 

 the margins. The exact position to which the former are at- 

 tached in relation to the lines of the blood-channels cannot 

 therefore be clearly defined. This, however, is an unimportant 

 point, since the aggregate action of the cilia, as indicated by the 

 setting of the current, may be easily proved. 



The epithelium which lines the flat surfaces of the laminae 

 forms undoubtedly a continuous membrane. It is consequently 

 impossible that there can take place any water-currents between 

 the individual blood-channels, such as those which figure so 

 prominently in the mechanism of the Lamelhbranchiate gills. 

 The water-passages being wanting, it results that, in this Gas- 

 teropod, the branchia is not penetrated by the aerating medium. 

 This fact should be regarded rather as a criterion of superior 

 than of inferior organization. It signifies an increasing subdi- 

 vision of the blood- streams. The blood-vessels being smaller, 

 and the parietes being less dense and cartilaginous, exposure to 

 the oxygenating medium on one side only suffices for the pur- 

 poses of respiration. It is important in this place to remark, 

 that if the interlaminar space of the branchial leaves of Patella 

 could be proved to be filled with water and not with blood, some 

 ground would be afforded, as will again be shown, for supposing 

 that in these Mollusca the external water is actually admitted 

 into the penetralia of the organism, forming an aquiferous system, 

 and that it circulates like a nutritive fluid throughout the entire 

 body. This, however, is a fancy as yet at all events totally un- 

 supported by fact. 



It is difficult to leave this part of the present inquiry without 

 once more drawing attention to the homologous significance of 

 the branchiae of Patella, They are composed of two layers of ves- 

 sels, opposed face to face, and joined at the margin. They are 

 invested by a ciliated epithelium which is restricted to the exterior 

 aspect of the organ. The constituent layers are separated by an 

 intermediate space. In these several items of mechanism these 

 branchiae approximate most wonderfully closely to the type 

 which is normal to the Acephala. If there be no meaning in 

 this approximation, there can be no unity in the organic system of 



Ann. &; Mag. N. Hist. Ser. 2. Vol.xyi. 28 



