70 LAMELLIBRANCHIA. 



In Mytilus, as in some other Larnellibranchs (e.g., Pecten, Area) the gills, 

 even in the adult, consist of individual filaments which, however, are arranged 

 in just the same way as the branchial leaves of other forms. The inner row 

 becomes bent inward to form the ascending lamella of the inner leaf, while, 

 in the case of the outer leaf, the filaments are bent outward (Fig. 29 E). A 

 section of these gills has the form of a W, and thus resembles a section of the 

 gill-leaves in the Eulamellibranchs (Fig. 80 E). The free ends of the fila- 

 ments seem to be connected by a continuous strand of tissue running parallel 

 to the length of the gill-leaf. This latter must be regarded as the modified 

 representative of that transverse connection found uniting the free ventral 

 ends of the papillae when the gill first arose, shifted dorsally. The papillae 

 themselves correspond to the filaments of the adult gill. Since, in Mytilus 

 also, the reflected or ascending portion of the gill is at first represented by a 

 solid plate (see the above description of the development of the gill) in which 

 the slits arise secondarily, the Mytilus gill, in its later stages, passes through a 

 condition resembling that seen in the earliest gill-rudiment in such Larnelli- 

 branchs as Cyclas and Teredo [or better still, in Pholas, Singerfoos], the 

 gills of which originate as leaves. There is therefore some difficulty in 

 regarding, with many authors, the later filiform condition of the gill as an 

 original condition. This difficulty is increased by the fact that the gills of 

 Mytilus, Pecten, etc., which consist of single filaments, have, when regarded 

 as a whole, the general characters of a branchial leaf with descending and 

 reflected or ascending lamellae, the descending and ascending limbs of the 

 same filament being united together by fusions of tissue at certain points, the 

 so-called interlamellar junctions ; further, the adjacent filaments of the same 

 row, both in the ascending and descending limbs, are held together by the 

 interlocking of some specially long cilia. Wherever, therefore, we have gills 

 consisting of independent but reflected filaments, the assumption that these 

 filaments might have arisen by a secondary separation of the gill-bars in a 

 primary branchial plate is suggested (p. 1'2). 



It appears that the papillae correspond to the gill-bars or, as they 

 are generally termed, filaments of the adult and the slits to the 

 interstices between these bars. The differentiation of the bars would 

 then have to take place from the posterior end of the gills. The gill 

 of the adult Lamellibranch is usually a much more complicated struc- 

 ture than the larval gill up to the stage we have described. Between 

 the filaments of each lamella, as well as between the ascending and 

 descending lamellae of each leaf, there are connections which may 

 consist of solid cell-strands, of hollow vascular junctions, or simply 

 of interlocking cilia, so that the leaves are connected by longitudinal 

 interfilamentar and by transverse lamellar junctions. The mesoderm 

 of the papillae yield the connective tissue, the blood-vessels and the 

 skeletal rods which support the gill-bars, and from thence extend 

 in certain forms into the complicated junctions found in most 

 Larnellibranchs (Eulamellibranchs and Pseudolamellibranchs). 



