LAMELLIBRANCHIATA. 



339 



obvious, but both the descending and ascending limbs of adjacent 

 filaments are connected so as to form a kind of trellis-like mem- 

 brane, the ascending limbs into one membrane and the descending 

 limbs into another. Between the porous membranes ^o formed 

 is contained a space, which communicates with the general mantle- 

 chamber by the 

 pores left be- P 



tween the fila- 

 ments where the 

 latter have not 

 undergone these 

 interfilamentar 

 concrescences. 



In this way 

 the Eulamelli- 

 branch condi- 

 tion, in which 

 there appear to 

 be on each side 

 two gills, each 

 composed of two 

 lamellae, is ar- 

 rived at. A very 

 similar condition 

 is presented by 

 the Filibranchs 

 and some Pseudo- 

 lamellibranchs, 

 the difference 

 being that in 

 these groups the 

 lamellae of the 

 gill break up at 

 the slightest 

 touch into their 

 constituent fila- 

 ments, because the filaments are only held together by the inter- 

 locking of cilia; whereas in the former the lamellae are coherent, 

 because the filaments (which are still perfectly obvious) are held 

 together by continuity of tissue. Further, in the Eulamellibranchs 

 the outer lamella of the outer gill has undergone concrescence with 



FIG. 267. Pieces of transverse sections through the branchiae of 

 Anodonta (from Lang, after Peck). A outer, B inner gill. In 

 each section the two lamellae are shown as connected together 

 by the interlamellar concrescences, which are formed by the 

 sub-filamentar vascular tissues of, in B. The interfilamentar 

 concrescences are effected by the same tissue. In C, which is 

 a part of B more magnified, this tissue is marked il, lac, and ol ; 

 and the filaments with their frontal epithelium and their chit ; - 

 nous tissue ch, and rods chr. ol outer lamella ; il inner lamella ; 

 v blood vessel ; of subfllamentar tissue ; /filaments ; ch chitinous 

 tissue of filaments ; chr specially condensed chitinous rod in ch ; 

 lac subfilamentar tissue. 



