272 



MOLLUSC A. 



On either side of the foot there hang the gill-lamellce^ oi 

 cteiiidia. These are lamellae on each side, formed in each 

 Respiratory ^^^^ ^^ ^ gill-plate folded on itself, the outer 

 gill-plate outwards and the inner inwards. The 

 gill-plates are themselves composed of a number of gill- 

 filaments^ which hang perpendicularly in a single row from 

 a horizontal axis which is fused with the body-wall. 



Fig. 191 — View of Anodonta with Left Mantle-Flap thrown 

 Back. {Ad naf. ) 



Left Mantle Flap. 



Inhalent 

 Aperture. 



Outer Gill. 

 Renal Inner Gill. 



Foot. Labial Genital Aperture. 



Palp. Aperture. 



A ctenidium therefore consists of a medium axis with 

 two rows of gill-filaments, each row forming a gill-plate. In 

 Anodonta. but not in all Lamellibranchiata^ these gill-plates 

 are bent double to form in each case two gill-lamellse. In 

 addition, the filaments and the gill-lamellse have fused with 

 their fellows and thus form a network of filaments. The 

 whole are ciliated and cause currents of water and food- 

 particles to pass into the mantle-cavity by the inhalent 

 aperture. The free edges of the upturned gill-plates are 

 fused to the body-wall, and thus shut off outer and inner 



