306 ROBERT TRACY JACKSON ON THE 



ing the reilectod jioi-tion and of later growth. This will be better understood by com- 

 paring the diagrams of the gills of Ostrea, figs. G-9, with those of Anomia, fig. 11, and 

 Pecten, PI. xxiv, fig. 12, where the origin and relation of separate gills and gill folds 

 are perfectly clear. The newly formed gill B' was composed of the same number of 

 filaments as was the well developed gill A'. In dissecting similar specimens it was 

 found that the tips of the filaments of the gill B' were already folded back upon them- 

 selves, but they are reflected outwardly instead of reflected inwardly (toward the median 

 line of the bod}') as is the case with the filaments of the first formed gills (again com- 

 pare with the diagram of gills of Pecten and Anomia). After an interval of two days, 

 a fourth gill, B, shown diagrammatically in PI. xxtv, fig 8, appeared on the left side on 

 the basal mai-gin of A. It began as a minute bud-like process and grew as already 

 described for gill B'. 



In other specimens up to 4.75 mm. in height, a third gill, B', had begun to develop on 

 the right upper side as in PI. xxiv, fig. 4, but no fourth gill had yet appeared on the left 

 lower side. Oysters of 1 cm. in height had all four gills developed, but in dispropor- 

 tionate degrees. The right outermost was short, the right innermost long, the left in- 

 nermost longer still, and the left outermost (the last formed) very short, as in the 

 diagram, PI. xxiv, fig. 8. 



First, it is noteworthy that those gills, which are to form the right and left outermost 

 gills of the adult, appear much later than the two innermost gills. Secondly, it is a re- 

 markable and interesting fact that a considerable time elapses between the origination of 

 the right outermost gill B' and the left outermost gill B. Stoliczka, in his "Cretaceous 

 Pelecypoda," on page 2, says, "I have examined one or two animals of almost every fam- 

 ily of the Pelecypoda, and in all I found that there are at least two gill-lamell;p present 

 on either side, — never a single one, though they are often unequal and one of them some- 

 times, as in the Tellinidae, nearly obsolete."^ The late appearance of the third and fourth 

 gills in Ostrea may have phylogenetic significance, but the irregularity of their ajjpearanee 

 is probably due to other causes, perhaps due in some way to the pressure exerted on the 

 parts as the oyster lies on the left side in its adopted habit of fixation. 



Viewing our specimen from the left side through the glass to which attached, it was 

 found that the dorsal margins of the several gill-lamellfe Avere not superimposed; the 

 united borders of gill B' and the outer lamella of gill A! were near the middle line of 

 the valves. The reflected borders of A* and A united to form a margin nearer the ven- 

 tral edge of the shell than the previous gill margin. The outer lamella of ^ and the new 

 gill B, when formed, made a united border still nearer the edge of the shell. The result of 

 this want of juxtaposition was that, when viewed from the right side, fig. 4, PI. xxiv, the 

 gill A looked much larger than A!, because it extended nearer to the margin of the 

 shell. In degree this was untrue, though as a matter of fiict A is somewhat larger 

 than jH. Dr. Pelsener, in his report on Lamcllil)rancliiata in the Challenger Reports, 

 gives a very clear figure showing the condition and relation to one another of the gill 

 filaments in a lai'ge numl)er of genera. 



The course of the aliineiilary tract has been (raced in the young oyster by Pyder (03), 



' According to Hall (Ifi), a single gill-lannlla. cciniposcd l)iicfly discusses Uie relations of the gills and classification 



of si'iiaratcd filaments, exists on cither sidi- in Diniya. He on tliis Ijasis in I5nll. Mns, Conip. Zoijl.. Vol. xviii, 1S89. 



