SEA MUSSEL MYTILUS EDULIS. 



I6 5 



ascending and descending limbs, varying in number from 15 to 30, according to the 

 age of the specimen (figs. 148, CJ, p. 164; figs. 150, and 151, CJ). 



The ascending and descending fibers of each gill are attached by cross partitions 

 of tissue which have the form of bars. These structures were given the name inter- 

 lamellar junctions by Peck (1877). (See fig. 148, //.) They are usually three or four 

 in number, separated by some distance and not grouped as Peck figures them. 

 According to this author they are composed of longitudinal elastic or muscular fibers 



'-LFC 



FlO. 149.— Transverse section of gill filament taken through the area between ciliary junctions. X 500. 

 Fig. 150.— Transverse section of two gill filaments taken through a ciliary junction. X SCO- 

 Abbreviations. — AFC, abfrontal cilia: BC, blood corpuscle; BrV, branchial vein; CJ. ciliary junction; FC. frontal cilia; 

 LFC, latero-frontal cilia which lash in direction indicated by arrow; LC, lateral cilia which lash in direction indicated by arrow. 



surrounded by a layer of epithelium which gives them great powers of extensibility 

 and contractibility. He speaks of them as "bellowslike processes." They are trav- 

 ersed by a canal which allows cross communication between the vessels of the ascending 

 and descending limbs. The walls of the latter canals, according to Peck (1877), are 

 lined with chitin, while those of the interlamellar junctions are not. Kellogg (1892) 

 makes a similar statement but in addition says an endothelial lining is present. What 

 these authors refer to as chitin is probably not that substance but conchiolin or some 

 related compound. 



