GILLS OF LAMELLTBRANCH MOLLUSCA. 47 



outer plate on either side is known as the right or left outer 

 gill-inlate. The inner phite is similarly known as the right or 

 left inner gill-plate. Each plate consists of a double row of 

 exceedingly delicate gill-filaments, placed side by side and 

 absolutely parallel to one another. If we examine one gill- 

 plate closely by separating the parallel filaments, we find 

 that the double roAv is not formed by two sets of filaments 

 attached along the lophophoral line; but that for each gill- 

 plate there arises from the lophophoral line but a single row 

 of parallel gill-filaments^ each one of which (the ant-uni- 

 bonal surface of the Lamellibranch being its tentral surface), 

 after descending in a ventral direction, suddenly bends upon 

 itself at an acute angle, and passes upwards again, forming 

 the second row of filam.ents which constitutes the plate. M. 

 de Lacaze Duthiers has observed that this is the actual mode 

 of growth of the gill-filaments in Mytilus. Each gill-plate 

 thus consists of a single row of filaments bent on itself, so as 

 to form two rows of filaments or lameUcB, with a space 

 between them. One lamella is formed by the descending 

 portions of the filaments, the other by the ascending portions. 

 The ascending portions of the filaments are not attached to 

 the lophophoral line, nor (in Mytilus) to the mantle. The 

 ascending portion of each filament terminates in a hook- 

 shaped process, which is closely adherent to the similar hook- 

 shaped process of the two adjacent filaments, and probably in 

 vascular continuity with them. Thus the ascending lamella 

 or portion of the filaments is free in the region of the base 

 or lophophoral line ; the descending lam.ella or portion of the 

 filaments, on the other hand, is here at its fixed point of 

 origin. The outer gill-plate of each side of the animal has 

 the free or ascending lamella placed outermost/ or nearer 

 the mantle; the inner gill-plate of each side of the animal 

 has the free or ascending lamella placed innermost, or nearer 

 the foot. Thus two complete filaments, one of each of the 

 two gill-plates of one side of the body (say the left) seen in 

 position as exposed by a cut dividing the mussel vertically 

 into an anterior and posterior moiety, have the appearance of 

 the letter W. The broad upper part of the letter is the base 

 of the two gill-plates ; the two angles on which the letter 

 stands are respectively the apex of the inner (medial) and 

 outer (lateral) gill-plates. The central angle at the base of 

 the letter is the point of junction for each gill-plate with the 

 lophophoral ridge. From this descends on the left-hand side 

 the filament of the inner lamella of the outer gill-plate; it bends 

 on itself, forming the fl'^;ea:oraw<//e of that gill-plate, and ascends 

 as tlie filament of the outer lamella of the outer gilUplate ; it 



