GILLS OF LAMELLIBRANCH MOLLUSCA. 5 7 



Straight from base to apex ; the transverse element furnished 

 by the sub-filanientar vessels is of an undulating character, 

 though in its main direction truly at right angles to the 

 filaments. 



Langer first showed by injection the presence of the large 

 vertical vascular trunks and their horizontal branches. He 

 was erroneously led to believe that the horizontal branches 

 lead into a closed capillary system. As a matter of fact 

 they do not, but open by a loosening or incoherence of their 

 walls into the general space (continuous with the body 

 cavity), which is the characteristic feature of the lacunar 

 tissue, which forms the substance of the sub-filamentar out- 

 growths. 



The horizontal vessels are often very ill-defined and of 

 short extent, but here and there can be demonstrated in con- 

 nection with the large vertical trunks (PI. V, fig. 11 v'). When 

 ill-defined they can yet be identified as more or less sharply- 

 marked pathways or clear spaces in the lacunar tissue. Dr. 

 Posner was the first to show by microscopic sections the con- 

 tinuity of Langer's vertical vessels with the surrounding 

 lacunar tissue and the absence of a capillary system with 

 definite walls. But he seems to have gone too far in identi- 

 fying Langer's vertical and horizontal trunks entirely with 

 the lacunar tissue, the spaces and trabeculee of which he 

 figures and describes. It appears from my sections (as well 

 as from Kollmann's recent observations) that there are very 

 definite walls, formed, one may say, by a condensation of the 

 lacunar tissue, to the large vertical trunks of Langer, and 

 there are even definite walls to the origins of the horizontal 

 blood-pathways (see Plate V, fig. 11). 



The layer of sub-filamentar tissue which occurs below 

 each lamella of each gill-plate is formed by the concrescence 

 (inter-filamentar) of the excrescences or bulgings-out of the 

 deepi border of each gill-filament. The inter-filamentar 

 concrescence of these outgrowths is very complete, and such 

 as to leave only a series of undulating rows of small cylin- 

 drical passages opening by stomata or windows on the outer 

 and on the inner surface of the lamella (Plate V, figs. 5 and 

 8). These narrow passages, which are the only parts of the 

 interfilamentar spaces not obliterated by the concrescence of 

 the sub-filamentar outgrowths, have a cylindrical shape, and 

 run somewhat obliquely and irregularly. The horizontal 

 elements of Langer's vascular system correspond with, and 

 are excavated in, the continuous tracts of lacunar tissue which 



' Deep in the seuse of remote from the free outer surface of the gill- 

 lamella. 



