680 



ZOOLOGY 



SECT 



and inner gill-laminae, and left outer and inner gill-laminae (Fig. 

 565, l.ext. gl, lint. gl}. Seen from the surface, each lamina 

 presents a delicate double striation, being marked by faint lines 

 running parallel with, and by more pronounced lines running at 

 right angles to, the long axis of the organ. Moreover, each 

 lamina is double, being formed of two similar plates, the inner and 

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

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

 has thus the form of a long arid extremely narrow bag open above 

 (Figs. 566, 567 and 568) : its cavity is subdivided by vertical bars of 

 tissue, the inter-lamellar junctions (i. l.j.}, which extend between the 



rp.u. 



II 



cx.spJt, 



in.sph 



w.l, 



FIG. 566. Anodonta cygnea. Dissection from the left side. a. anus; a. ad. anterior 

 adductor ; a. ao. anterior aorta ; a. <\ ap. auriculo-ventricular aperture ; bl. urinary bladder ; 



c. pi. yn. cerebro-pleural ganglion ; d.d. duct of digestive gland ; d. ill. digestive gland ; 



d. p. a. dorsal pallial aperture ; ex. sph. exhalant siphon ; ft. foot ; g. ap. genital aperture ; 

 i/oii. gonad ; gul. gullet ; i. 1. j. inter-lamellar junction ; in. spit, inhalant siphon ; int. intes- 

 tine ; kd. kidney ; m. mantle ; mth. mouth ; p. ao. posterior aorta ; p. ad. posterior adductor ; 

 pc. pericardium ; pd. fin. pedal ganglion ; r. ap. renal aperture ; r. au. right auricle ; 

 ret. rectum ; /. p. a. reno-pericardial aperture ; st. stomach ; ty. typhlosole ; c. ventricle ; 

 r. (jn. visceral ganglion ; w. t. water tubes. 



two lamellae, and divide the intervening space into distinct compart- 

 ments or water-tubes (w.t.), closed ventrally, but freely open along 

 the dorsal edge of the gill. The vertical striation of the laminae is 

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

 set gill-filaments (/.) : the longitudinal striation to the circumstance 

 that these filaments are connected by horizontal bars, the inter- 

 filamentar junctions i. f.j.). At the thin free or ventral edge of the 

 lamina the filaments of the two lamella are continuous with one 

 another, so that each lamina has actually a single set of V-shaped 

 filaments, the outer limbs of which go to form the outer lamella, 

 their inner limbs the inner lamella. Between the filaments, and 

 bounded above and below by the inter-filamentar junctions, are 



