58 R. HOLMAN PECK. 



separate these cylindrical passages. The stomata, hy which 

 the passages, " water-passages " as they may well be termed, 

 open to the water outside the gill-plate at the one end, and 

 to the water in the inter-lamellar space on the other, are seen 

 in Plate V, figs. 14, 17. They are not set in strictly hori- 

 zontal rows, but vary in position above and below a mean 

 horizontal line. The diameter of the stomata on the deep 

 surface of each lamella is considerably smaller than that of 

 the respective toater-passages into which they lead, whilst the 

 ])assages do not contract, but become a little elongated at 

 their termination between the filaments on the free surface 

 of the lamella, forming thus a series of somewhat oval 

 stomata at that end. The absolute size of the stomata, and 

 of the lumen of the water-passages, as seen in section, varies 

 considerably, the latter measurement giving --y',;^th to -a-yo^th 

 of an inch. 



The result of the narrowing (often abrupt) of the water- 

 passages at the interlamellar surface is that in a transverse 

 section, such as figs. 1, 2, 3, Plate V, a water-passage may be 

 seen between nearly every adjacent pair of filaments ; many 

 of which — sometimes all — appear closed at the deep or inter- 

 lamellar surface of the section by a continuous tract of la- 

 cunar tissue. The real state of the case is shown in the 

 drawing of a solid block of gill-lamella, given in Plate V, 

 fig. 8. 



Interlamellar junctions. — At definite intervals the two 

 lamellae which form the gill-plate of Anodon are joined by 

 outgrowths, constituting interlamellar junctions. These 

 differ from the tubular bellows-like interlamellar junctions 

 of Mytilus, in that they are not scattered cylindroidal out- 

 growths of the deep margin of individual filaments, but are 

 long ridges running vertically, parallel with the filaments ; 

 they are rather additional outgrowths of the sub-filamentar 

 mass of tissue beneath each lamella than outgrowths from 

 individual filaments. They are coincident in position with 

 the large vertical blood-vessels, and differ as do the latter in 

 the inner and the outer gill-plates. 



In the case of the inner gill-plate the large vertical vessels 

 are disposed at intervals of about twenty filaments. Each vessel 

 is found to lie alternately (not without exception) in the inner 

 and the outer lamella, forming a large vertical ridge on the 

 internal surface of the lamella (Plate V, fig. 3). These ridges, 

 alternately belonging to one or other of the two lamellas, 

 unite very intimately by concrescence with the lamella to 

 which they do not originally belong. Sometimes the union 

 is more, sometimes less complete. The result is that the 



