352 



THE AMERICAN NATURALIST [Vol. XLIV 



as thickness, length, and the greater freedom of the right 

 side from the body, where, for a considerable area in 

 front of the adductor muscle, there is open communica- 

 tion from the supra-branchial chamber to the outside at 

 the dorsal edge of the shell (Figs. 14, 15). 



The anterior adductor muscle is present for a time in 

 the spat, and appears to move upwards and backwards 

 from its original position in the larva, until it is crowded 

 to the edge and disappears before the oyster reaches 1 

 mm. in depth. The posterior adductor muscle, on the 

 other hand, increases regularly in size and moves down- 

 wards and slightly backwards, leaving distinct lines on 

 the shell to indicate its change of position. In spat .86 

 mm. high it is just below the edge of the prodissoconch 

 (Fig. 2), in those 1.5 mm. high it is twice the depth of the 

 prodissoconch from the top of the shell (Fig. 3) ; in all 

 stages it is nearly in the median horizontal plane and 

 slightly posterior to the median frontal plane. The 

 movement may be effected by a slow creeping of the 

 muscle, due to downward pressure from the growth of 

 the body above, or by the addition of new fibers below 

 and the absorption of old from above, while the impres- 

 sions on the shell may result from the inability of the 

 mantle to deposit new layers of pearly matter under the 

 attached ends of the muscle. 



Of the organs enclosed by the mantle a conspicuous 

 part is early assumed by the gills. The oldest larva or 

 the youngest spat has two of these— one on each side of 

 the body, below and between the line of attachment of 

 the mantle and the base of the foot. At this time each 

 gill possesses a row of about eight filaments, of which 

 the anterior are larger and more completely separated 

 from each other, the posterior are shorter and only partly 

 separated by vertical creases, while all are united above 

 by an undivided longitudinal ridge or basal axis that 

 projects behind and below the mass of the body. At the 

 time of fixation the right and left gills are approximately 

 equal, but when the spat reaches .86 mm. in height (Fig. 



