XII 



PHYLUM MOLLUSCA 



637 



a right and a left outer (Fig. 527, /. cxt. gl), and a r'njlit and a left 

 inner gill (1. int. gl.). Seen from the surface, each gill 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 gill is 

 double, being formed of two similar plates, the inner and outer 

 /"iitcllce, 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 narrow bag open above (Figs. 528, 

 529 and 530) : its cavity is subdivided by vertical bars of tissue, 

 the inter-lamellar junctions (i. l.j.), which extend between the two 



in.sph 



IV.l 



FIG. 528. Anodonta cygnea. Dissection from the left side. . anus ; u. <l. anterior 

 adductor ; a. ao. anterior aorta ; a. r. ap. auriculo-ventricular aperture ; l>l. urinary bladder ; 

 c. pi. gn. cerebro-pleural ganglion; <'. >'. duct of digestive gland; d. fil. digestive gland; 

 i!. /*. a. dorsal pallial aperture ; ex. sph. exhalant siphon ; ft. foot ; g. ap. genital aperture ; 

 no it. gonad ; nv.l. gullet ; i. /. j. inter-lamellar junction ; in. spli. inhalant siphon ; int. inte>- 

 tine ; k<l. kidney ; /. mantle ; mth. mouth ; p. ao. posterior aorta ; p. ml. posterior adductor ; 

 pc. pericardium ; p'L gn. pedal ganglion ; r. np. renal aperture ; r. an. right auricle ; 

 ret. rectum : /. p. a. reuo-pericardial aperture ; st. stomach ; t>/. typhlosole ; r. ventricle ; 

 r. 11,1. visceral ganglion ; v:. t. water tubes. 



lamellae, and divide the intervening space into distinct compart- 

 ments or water-tubes (u\ 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-filaments (/.) : the longitudinal striation to the 

 circumstance that these filaments are connected by horizontal 

 bars, the inter-filamentar junctions (i. f. /.). At the thin free or 

 ventral edge of the gill the filaments of the two lamellae are con- 

 tinuous with one another, so that each gill 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 



