54 R. HOLMAN PECK. 



junction, otherwise we should find appearances similar to 

 those given in the upper part of fig. 6 (Mytilus). 



Beneath the epithelium is a thin layer of chitinous sub- 

 stance coloured yellow in the figures. This is formed by the 

 modification of the connective tissue (lacunar tissue). In 

 the case of Area and of Mytilus I have not ascertained the 

 presence of protoplasmic corpuscles, or nuclei embedded in 

 this thin chitinous layer, but on its inner surface which 

 bounds the cavity of the filament, there are such corpuscles 

 which give rise to septal filaments (sep.) limited in Area to 

 one series. These traverse the cavity of the filament and 

 divide it into two chambers, a lateral and a medial. Possibly 

 the two juxtaposed canals thus formed may have a func- 

 tional importance. 



Mytilus. — The general form and arrangement of the gill- 

 filaments constituting the gill-plates in the common edible 

 mussel are familiar to most zoologists, and are well figured 

 in the last edition of Cuvier's 'Regno Animal.' Like Area, 

 Mytilus has the ends of the reflected portions of its gill-fila- 

 ments free. Like those of Area, the adjacent filaments are 

 held together by ciliated junctions. But there is a remark- 

 able exhibition of concrescence in Mytilus which I have not 

 met with in Area, consisting of the sub-filamentar outgrowths 

 •which I call " interlamellar junctions." In PI. IV, fig. 10, 

 is represented diagrammatically (the drawing being shortened 

 in proportion to breadth) a single filament of the outer gill- 

 plate of the left side. The filament of the inner lamella 

 (/?/.) is seen descending from the base to the apex or angle of 

 the gill-plate and returning on itself as the filament of the 

 outer lamella, or reflected filament where it ends in a hook. 

 The intcrlamellar junctions are represented as they occurred 

 in a particular specimen. They are not U7iiformly in the 

 position given in the figure, nor grouped precisely in the 

 way there indicated throughout the series, but they run in 

 irregular transverse rows alo7ig the series of filaments. Some 

 of these intcrlamellar junctions are very long, and appear to 

 have a great extensibility. When not extended they have 

 the appearance of a bellows being deeply folded in their 

 walls. This folding admits of their extension to a relatively 

 enormous length. In fig. 9 one of these bellows-like pro- 

 cesses is drawn in a state of semi-extension. Beneath its 

 epithelium longitudinal fibres either elastic or muscular, I 

 am unable to decide -which, are seen. In fig. 8 parts of the 

 descendingand ascending limb of a gill-filament are seen. Each 

 carries three epithelial prominences, ep. (ciliated junctions), 

 and between them two intcrlamellar junctions {ilj .) pass in a 



