250 Dr. T. Williams on the Mechanism of Aquatic 



valent to an entire single gill. As formerly intimated in the 

 example of a single gill, the limbs of the same looped bar, 

 respectively venous and arterial, are placed on different hori- 

 zontal planes (PL VII. fig. 14), the planes of the loops (a) at 

 the free margin being vertical, and not horizontal as they are in 

 general in the double gill (PI. VI. fig. 1 ; PI. VII. figs. 9 & 11). 

 The single gill, like the double, is composed of two lamellar 

 planes (fig. 14 b, c) bounding intermediate water-tubes. But 

 in the single gill each lamella is single in function, since it 

 consists of the afferent or efferent limbs separately and exclu- 

 sively of the looped bars. In either lamella therefore the 

 adjacent bars belong to separate and independent loops. The 

 component bars of the lamellse in all single gills are separated 

 from one another by intervectal water-fissures (PI. VII. fig. 15 

 c, c, c). In the double gills in which the two limbs of the same 

 loop lie adjoined on the same horizontal plane, such limbs are 

 united together by a continuous membrane (PI. VI. fig. 5). In 

 such case the intervectal water-fissures exist only between the 

 limbs of different contiguous loops, not between those of the 

 same looped bar. By this arrangement the volume of water 

 which traverses the gill at any given time is reduced by exactly 

 one-half. The functional value of the organ therefore sinks 

 in the same degree. A double gill (PI. VII. figs. 13, 9 & II ; 

 PI. VI. fig. 1) in structure is not necessarily twofold in physio- 

 logical import. In official activity it exceeds little the single gill. 

 In the latter the blood is more intimately brought into contact 

 with the respiratory medium, and this medium is more readily 

 and rapidly renewed. To the single gill (PI. VI. fig. 2 ; PI. VII. 

 fig. 14) conchologists have applied the term supplementary. It 

 is difficult to understand in what sense this term should be re- 

 ceived. In structure the single gill is not supplementary. It is a 

 perfect and complete organ. No constituent element is deficient 

 or suppressed. In function it is complete. It is not a super- 

 numerary organ. Both these designations are significant of 

 what is untrue. It is as much an integer of the organism as the 

 upper or inner gill. A law hitherto undiscovered does, however, 

 affect the presence and dimensions of the outer or single gill 

 which does not influence the inner or double gill. If, as in the 

 Pandoridse, Lucinidse, and some other families, there exist only 

 one gill, it is invariably the single or out-gill that is wanting. 

 The principle of suppression or non-development affects exclu- 

 sively the latter. When only one gill exists, that is, one on 

 cither side of the foot and body, it is always double in structure. 

 It contains the same number of bars and loops as any other 

 double gill. It is quite erroneous to conceive that in such a 

 case the absent or suppressed gill has been fused into and iden- 



