260 Dr. T. Williams on the Mechanism of Aquatic 



receiving the blood of two separate lamella, not that of the same 

 lamella as in the case of the double gill ; h, h, bars of the upper 

 lamella ; f, those of the lower ; e, connecting fibrous threads 

 tying together the bars ; d, the bars of the lower lamella, i, j ; 

 c, c, the interlamellar framework as explained in the double gill. 

 g Sf f, show the single row of loops of which the free margin of 

 this single gill is composed : but here the plane of each loop is 

 placed vertically, not horizontally as in the double gill. 



Fig. 3. A diagram representing the arrangement of the loops, a, a, c, d, at 

 the free margin of the gill in some species ; b, is the water-chan- 

 nel conveying a current towards the mouth. The loops are placed 

 on vertical planes. Each lamella in this case is composed of 

 afferent or efferent vessels exclusively. 



Fig. 4. Two loops from the free margin of the double gill of Pholas, show- 

 ing the mode in which the continuous membrane, a, a, which in 

 some bivalves, if not in Pholas, really forms the blood-channel. 

 In such cases the bars, b, b, c, c, would be solid. The water- 

 stigmata (intervectal orifices), d, are in such instances present only 

 between alternate pairs of bars. 



Fig. 5. Plan representing the manner in which the bars, a fy b, of two se- 

 parate lamellae placed on the same vertical planes, but different 

 horizontal planes, are tied together by vertical partitional perfo- 

 rated membranes (e) ; d, shows the water-orifices in that con- 

 necting structure (e) which unites the bars of the same lamella 

 horizontally. 



Fig. 6 — 6 2 . Minute anatomy of the branchial bars in those cases in which 

 the hyaline cartilages, a, a, are so closely approached as to form 

 the boundary of the channel (c) ; b, b, pieces of the cilia-bearing 

 epithelium stripped off. 



Fig. 7- View of a piece of the entire gill of Thracia convexa. The free, 

 distal margin is turned upwards, a, b, the double rows of verti- 

 cally disposed loops of which this margin is made up ; g, the con- 

 traction which occurs between the interlamellar water-tubes (e) ; 

 /, one of the transverse pieces of the interlamellar framework 

 in view, crossing the water-tube, indicated by the arrow. 



Fig. 8. The same cut horizontally along the length of the bars. The free 

 edge (b, c) of the upper lamella is left, d, d, show the mode in 

 which the transverse pieces of the intra-tubular framework lie 

 between the lamellae, a, b, which constitute the walls of those 

 tubes ; e, e, arrows denoting the direction of the water-currents 

 in the tubes. 



Plate VII. 



Fig. 9. A small piece of the free edge of the double gill of Venus striatula. 

 a, b, loops of the bars of the two component lamella? lying hori- 

 zontally ; the loops lobed (fig. 10) as they are in Mytilus. c, the 

 afferent and efferent bars of a single loop. The arrow descends 

 from the interlamellar water-tubes. 



Fig. 10. One of the loops from fig. 9, enlarged, representing the lines and 

 disposition of the vibratile cilia, a, b : c, is a fleshy nodide tying 

 the loop to its neighbour. 



Fit/. 11. The free margin of the gill of a minute freshwater bivalve. It is 

 produced with a view to illustrate the continuous membrane which 

 in some cases ties the branchial bars together at the free border of 

 the gill, a, b, the loops of the two lamella; are shown as if they 

 were separated by the intervening tube (c), but in nature the 



