GILLS OF LAMELLIBRANCH MOLLUSCA. 59 



interlamellar junctions in the inner gill-plate contain the 

 large vertical vessels. In the outer gill-plate this is not the 

 case. 



The interlamellar junctions in the outer gill-plate are, like 

 the vertical vessels, more numerous than those of the inner 

 plate, occurring at intervals of seven filaments. They are 

 long ridges of dense lacunar tissue, ruuning vertically from 

 base to apex of the gill-plate, and have a much greater size, 

 measuring more from one lamella to the other than those of 

 the inner gill-plate. In fact, they are capable of very great 

 extension, which takes place when the outer gill-plate has 

 its interlamellar space occupied by the Glochidian young 

 of the Anodon (Plate V, fig. 4). This great depth of the 

 interlamellar junctions of the outer gill-plate is their most 

 remarkable feature, as compared with those of the inner 

 plate. It is accompanied by a different disposition of the 

 vertical vascular trunks; for, whilst these in the inner gill- 

 plate lie in the interlamellar junctions, in the outer gill-plate 

 they lie in the sub-tilamentar mass of concreted tissue at 

 the line of origin of the great ridges which act as inter- 

 lamellar junctions. In consequence of this arrangement 

 there are tioo vertical vessels in the outer gill-plate to each 

 interlamellar junction ; whereas there is only one to each 

 junction in the inner plate. The arrangement of these parts 

 in the outer gill-plate is no doubt correlated with its function 

 as a brood-pouch. Histological changes go on in these parts 

 of the outer gill-plate throughout the year, varying accord- 

 ing to the presence, absence, or size of the Glochidia or ova. 

 I have not yet followed out these changes in detail, and 

 must point out that there is a certain amount of irregularity 

 as to the development of the vertical vessels, especially as to 

 the definiteness of their walls ; so that in some parts and at 

 some seasons they have well and sharply defined walls, 

 whilst at other seasons and in other parts of the same outer 

 gill-plate they appear rather as Posner holds them to be 

 generally, namely, mere vertically-running lacuna?, or exca- 

 vations of the lacunar tissue, which may in some cases have 

 very little definition. This part of the subject I consider to 

 require further investigation. It appears possible that the 

 great vertical vessels of Langer may develope and be oblite- 

 rated from season to season in the rapidly changing lacunar 

 tissue of the sub-filamentar outgrowths. 



Differences between the inner and outer gill-plates. — The 

 diff'erence just noted between the outer and inner gill-plates, 

 due to the frequency of interlamellar junctions and their 



