No. 2.] FILAMENTS IN THE LAMELLIBRAXCH GILL. 73 



The surface of the gill lamellae may show either of two types. 

 It may be smooth, all the filaments lying in one plane ; or, with 

 the requirement of greater respiratory surface, the lamellae 

 may be folded parallel to the filaments. The two lamellae of 

 one gill are always folded symmetrically, so that the section 

 of the gill perpendicular to the filaments assumes an outline 

 reminding one, in its extreme form, of a string of wooden 

 button molds. Both filamentous and lamellar gills are subject 

 to this folding. 



The filaments occupying the bottom of the reentrant folds, 

 the limiting filaments, are usually somewhat larger than the inter- 

 mediate filaments, and are often very much modified in form 

 as well as size. These filaments may be easily traced through 

 the whole height of the gill and afford a series of fixed points, 

 aiding materially in the study of the folded type of gill by means 

 of sections. 



From the above resume it is evident that fusion or concres- 

 cence of parts has long been recognized in the lamellibranch 

 gill. Even in simple filamentous gills (Mytilus, etc.) the tips of 

 the filaments fuse to form a continuous band along the margin 



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of the ascending lamella. In more complex forms it is a process 

 of fusion which transforms the rows of originally distinct fila- 

 ments into the characteristic lamellae. The fusion of the ascend- 

 ing lamellae with the mantle and body is also perfectly familiar. 

 But the particular type of fusion described below appears to have 

 escaped mention in the somewhat extensive literature upon the 

 lamellibranch gill. 



Even a hasty study of serial sections of the strongly folded 

 gill of Cardium cditlc or Batissa tenebrosa shows the somewhat 

 surprising fact that each fold contains a far larger number of 

 filaments in the upper portion of the gill than in the neighbor- 

 hood of the free margin, this being equally true of ascending and 

 descending lamellae. (The extreme case was noted in Cardium, 

 where the number of filaments in one fold increased from eight 

 to thirty.) Yet the limiting filaments are continuous through- 

 out, showing that there is no reduction in the number of folds 

 correlated with the increase in the number of filaments in each 



t 



fold. These observations appear at first sight irreconcilable with 



