252 CEYLON PEART, OYSTER REPORT. 



with seven fenest rat ions. In the youngest form of which I have cut sections the 

 fenestrated gill lamellae are not reflected, and at the sides of the foot the lamellae of 

 opposite sides are quite free from one another and from the body wall and foot. 

 Behind the foot the lower edges of the lamellae of opposite sides are united by a 

 band of connective tissue, and still further back the organic connection between the 

 lower ends is more complete ; a vascular connection is established, and at the extreme 

 hinder end of the gill, where fenestration is still in progress, the gill lamellae of the 

 two sides are blended in a mass of embryonic connective tissue channelled by 

 numerous irregular blood sinuses. It follows from the above description that, if we 

 speak of the bars between the fenestrae as gill filaments, they are at all stages of 

 growth organically united in longitudinal series at their lower ends, and as the 

 filaments assume their complete histological structure, the chitinoid-supporting 

 skeleton of the filaments forms a dorsal and a ventral arcade, the upper end of each 

 hollow chitinoid gill bar curving forward to unite with the bar in front of it, and a 

 similar connection is eventually established at its lower end. In young specimens, 

 however, the skeletal bars pass below into a mass of undifferentiated connective 

 tissue. As growth proceeds, this undifferentiated tissue at the lower edge of the 

 anterior part of the gill lamella grows out in the form of a membrane, and as it 

 grows the membrane is reflected along the sides of the foot and grows upwards, 

 becoming fenestrated as it grows, and eventually the upper edge of what we now 

 recognise as the reflected lamella becomes attached to the body wall along the line of 

 union of the foot and visceral mass, thus completing the separation between the 

 supra-branchial and infra- branchial chambers. It would, perhaps, be more correct to 

 say that, as the reflected lamella grows upwards, the fenestrae of the direct lamellae 

 extend into it. When the adult relations are established, the chitinoid skeletal bars 

 of the filaments form an arcade along the upper edge of the reflected lamella where 

 it is attached to the body wall. In those adult individuals in which the reflected 

 lamella is imperfectly developed or absent (and such individuals are not uncommon 

 in both the species under consideration), it would appear that there is an arrest of 

 development, and that the larval condition of the gill becomes permanent in the 

 adult. This arrest of development suggests that the gills of Jousseaumia are 

 degenerating. As may be expected from the order of formation of the gill fenestree, 

 the anterior gill filaments are the longest, and they decrease progressively in length 

 from before backwards. 



In the youngest specimens there are no interfilameutar junctions, but these are 

 added in the course of growth, and, as can readily be understood from a consideration 

 of the manner in which the gills are formed, the posterior filaments have fewer 

 junctions than the anterior, as has been described by BERNARD for Scioberetia. 



A few irregularly scattered interlamellar junctions are formed soon after or during 

 the growth of the reflected lamella. These interlamellar junctions are vascular, 

 whereas the interfilamentar junctions are non- vascular. 



