Respiration in Invertebrate Animals. 251 



tified with the present solitary gill. The latter is the same in 

 essential structure as if the single gill were present. 



In Pholadomya and Anatina, Professor Owen describes the two 

 branchial lamellae of either side as having been united to form 

 a single gill*. Valenciennes states that the solitary gill of the 

 family Lucinidse resembles that of Anodonta ; it is larger, and 

 formed of thicker and more prominent pectinations. Lucina Ja- 

 maicensis, L. tigrina, L. columbella, and L. lactea, are examples in 

 which only a single branchial organ exists on either side. The 

 solitary gill differs from the ordinary double gill only in apparent 

 characters. The free border is composed only of two rows of 

 loops ; but these loops are soldered together by an obvious longi- 

 tudinal band or cord, running in shape of a deep water-groove 

 from one end to the other of the free margin (PL VI. fig. 3 b). 

 It is this character which occasions the appearance of doubleness 

 and fusion. In the solitary gill of the Pandoridse and Lucinidse, 

 the pectinations t of the lamellae are coarse and large to the naked 

 eye. This circumstance is due to the greater size in these cases of 

 the interlamellar water-tubes. It is repeated, that the vascular 

 elements, in the solitary gills, are the same in number and disposi- 

 tion with those of any other double gill. If, in the example of the 

 solitary gill, the outer single gill were really organically united 

 to the inner double gill, an organ should result consisting at the 

 free border of three rows of vascular loops, two distinct systems 

 of parallel interlamellar water-tubes, four separate lamellae, three 

 layers of afferent and three of efferent bars ! Such, of mechanical 

 necessity, should be the anatomical characters of a gill which 

 owed its formation to the union of one already double to another 

 struck on the single plan. Such a monstrosity is not illustrated 

 in nature. It is a fabulous branchia, born of hypothesis. But 

 it may appear quite reasonable to explain the anomaly of a solitary 

 gill, on the supposition that it is the natural and necessary pro- 

 duct of the fusion of two single gills. A glance at the illustra- 

 tions depictive of the type of the latter, will at once convince the 

 mechanician that two single gills could not in any manner be 

 fused in order to make a double gill, — such a double gill, that 

 is, duplex in mechanism, twofold in function, as actually exists 

 in the real animal. Let two single gills be brought together 

 (PI. VI. figs. 7 & 8), — the water-movement and the ciliary action 

 would cease at once on the two adjoined, apposed faces. Thus 

 the power of each would be reduced by one-half. Two singles 

 united make a single ! Such is the clumsiness of human handi- 



* Forbes and Hanley, British Mollusca, vol. ii. p. 42. 



t It should be distinctly understood, that the word 'pectinations* is not 

 synonymous with an ultimate branchial bar, but with that set of bars which 

 form an interlamellar water-tube. 



