3SS 



THE MUSSEL 



the long axis of the organ. Moreover, each gill is double, 

 being formed of two similar plates, the inner and oidcr 

 lamella;, united with one another along the anterior, ventral, 

 and posterior edges of the gill, but free dorsally. The gill 

 has thus the form of a long and extremely shallow bag open 

 above (Figs. 94 and 95) : its cavity is subdivided by vertical 

 plates of tissue, the intef-laniellar junctions (/. /. / ), which 



Fig. 94. — Di.igram of the structure of the gill o^ Atiodonin. 

 The gill i.s made up of V-shaped gill-filaments (/") arranged in longitudinal series 

 and bound together by horizontal inter-Jilamentar junctions (z'.y.y) ^^■hich cross 

 them at right angles, forming a kind of basket-work with apertures, the ostia 

 (^os), leading from the outside and opening {os^ into the cavity of the gill. The 

 latter is divided by vertical partitions, the inter-lamellar junctions (:. /,_;) into 

 compartments or water-tubes (rf. t) which open also into the supra-branchial 

 chamber ; b. 7'. blood-vessels. (From Parker and Haswell's Zoology.^ 



extend between the two lamellae and divide the intervening 

 space into distinct compartments or ivater tubes (Figs. 93 and 

 94, w. t), closed ventrally, but freely open along the dorsal 

 edge of the gill. The vertical striation of the gill is due to 

 the fact that each lamella is made up of a number of close-set 

 gill-filatnents (Fig. 94, / ) : the longitudinal striation to the 

 circumstance that these filaments are connected by horizontal 

 bars, the inter-filanienfar Junctions (i.f.J). At the thin free 



