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THE AMERICAN NATURALIST [Vol. XLIV 



a remarkable difference in the apparent ontogenetic de- 

 velopment of the gills of the oyster as compared with the 

 recorded phylogenetie development in lamellibranchs. 

 This, like the larval asymmetry of the shell, may be un- 

 derstood as a short-cut towards the final structure. As 

 the first-formed, short, basal axes grow backwards they 

 carry with them a continuation of the body-wall, con- 

 necting them with one another across the median sagittal 

 plane and with the mantle at both sides, giving rise to 

 a very imperfect separation into branchial and supra- 

 branchial (or cloacal) chambers. The filaments are at 

 first short, solid, papilla-like outgrowths from the axes 

 —the older and larger anterior, the younger and smaller 

 posterior. They originate behind, but by formation and 

 growth of new ones those first formed become pushed 

 forwards. The at first small and shallow supra-branchial 

 chamber follows this movement and penetrates each fila- 

 ment from above, distending it laterally but not antero- 

 posterior^ because of the pressure of contiguous fila- 

 ments. This process continues until each filament is 

 severed into an outer and an inner section, the inter- 

 mediate portion becoming constricted through, but never 

 completely at the tip. In this manner each gill-leaf be- 

 comes split into outer and inner lamellae of similar struc- 

 ture, but the one the reverse of the other, with its parallel 

 half-filaments and intervening slits overhung by cilia. 

 Imperfect separation of the transverse median portions 

 of the opposed halves of some filaments gives rise to the 

 inter-lamellar junctions, while the imperfect separation 

 of the opposed sides of different but contiguous filaments 

 occasions the inter-filamentar junctions. The same proc- 

 ess can be observed later in the development of the right 

 and left outer gill-leaves as well as in' the terminal fila- 

 ments of any one of the four (Fig. 14). 



anatomy and of the development of different groups has led to the 



