Respiration in Invertebrate Animals. 415 



arrangement. The individual laminse of the series mutually 

 support one another by a more exact and rapid adaptation of the 

 vis-a-vis surfaces. They are less liable to fold and wrinkle by 

 the rapidly varying degrees of distension to which they are 

 exposed by the action of the margin-muscles of the mantle. But 

 the free border of each leaflet is further so constructed as to 

 realize a great degree of elasticity (figs. 7, 6). On looking 

 sideways, that is, directly down upon the edge of the leaflet, the 

 eye discovers with perfect clearness an arched or vaulted outline 

 (fig. 7 C'f fig. 6 e) formed by the curving round of the blood- 

 channels of one half or layer of the sheet, in order to run into 

 those of the other half (fig. 7 «, e, b). It is by this method of 

 viewing the object that the anatomist may convince himself that 

 each gill-plate is really composed of two distinct and separated 

 layers (figs. 6 & 7 «, b-a, b), united only at the margin (c, c), in 

 exact accordance with the pattern of the single gills of the Lamel- 

 libranchiate Acephala. 



The satisfactory determination of this point of structure is of 

 great importance in this inquiry. It possesses all the power of a 

 key as regards the after-stages of the investigation. So extra- 

 ordinary is the uniformity of the plan on which the respiratory 

 organs of the branchiferous Gasteropods are formed, that it may 

 be inferred with perfect certainty that what is clearly proved to 

 be true of one grade in the series will apply with essential 

 accuracy to all the others. Extremely difiicult therefore though 

 it may be to unravel the minute structure of the gill-laminae 

 of Patella, from the key-like power of the information thus 

 only to be acquired, it is worthy of all the patience which the 

 student can command. 



Two facts of structure are then determined: — 1st. That the 

 plate is composed of two layers ; 2nd. That those two layers are 

 united by a looped arrangement at the margin. Now it may be 

 proved with exact certainty, that each layer is composed of 

 straight or wavingly parallel channels (figs. 6 «, b, and 7 «, b) laid 

 on the same horizontal plane (fig. 6 a, as far as i), such that a 

 membrane is formed. Although these channels are far less indi- 

 vidually distinct than those of the gills of the Lamellibranchs, 

 they are unquestionably blood-vessels, united together into a 

 membrane-like series by delicate intervening fibres or mem- 

 brane. Various kinds of proofs might be adduced in support of 

 this interpretation. 



The cilia follow the outline of each vessel (fig. 7 a, b) in a 

 line-like manner. The vessels present a linear bulge like a tube. 

 They can be seen to be filled with rows of corpuscles, clearly 

 distinguishable from the fixed cells and epithelia which form the 

 solid substance of their walls. Traced carefully in the direction 



