THE GILL IN PELECYPODA 



167 



Fig. 78. — Transverse section of portion of 

 an outer gill plate of Anodonta, liiglily 

 magnified : il, inner lamella ; il' , outer 

 lamella ; ilj, interlamellar junctions ; r, 

 large vertical vessels. (After Peck.) 



united or ' concresce ' with the mantle on the exterior and 

 with the base of the foot on the interior side. The leaves of 

 each gill plate, which have thns l^ecome doubled (the gills being 

 apparently two instead of one on each side), are folded or crum- 

 pled, and the filaments are modified at the re-entrant angles of 

 the fold. {PsctidolameUihranchiata.) 



4. In all the remaining Pelecypoda, except class 5, in other 

 words, in the very large majority of families, the filaments are 

 either reflected, as in (3), or 

 simple ; but the process of con- 

 crescence is so far advanced 

 that the adjacent filaments are 

 always intimately connected 

 with one another in such a 

 way as to admit the passage 

 of tlie blood ; and the leaves 

 of each gill-plate (Fig. 76, C) 

 are united by cross channels in 

 a similar way. {Eulamelli- 

 hraTichinta.) 



5. In certain of the Anatinacea alone (Cuspidaria, Lyonsiella, 

 Poromya, Silcnia) the gills are transformed into a more or less 

 muscular partition, extending from one adductor muscle to the 

 other (Fig.76, D), and separating off the pallial chamljer into two 

 distinct divisions, which communicate by means of narrow slits 

 in the partition. (SejJtihranchiata.) 



Thus the process of gill development in the Pelccypoda 

 appears to lead up from a simple to a very complex type. In 

 its original form, at all events in the most primitive form known 

 to us, the gill is a series of short filaments, quite independent of 

 one another, strung in two rows ; then the filaments become 

 longer and doul)le back, while at the same time they begin to 

 show signs of adhesion, as yet only superficial, to one another. 

 In a further stage, the reflected portions become fused to tlu; 

 adjacent surfaces of the foot and mantle, while the interLimellar 

 junctions serve to lock the two gill-plates together ; finally, the 

 mere ciliary junction of adjacent filaments is exchanged for inti- 

 mate vascular connection, while the gill-plates as a wliole become 

 closely fused together in a similar manner. 



This theory of origin is strengthened by closer oljserva- 



