248 



COLLEGE ZOOLOGY 



..6 



RESPIRATION. The respiratory organs of the mussel are the 

 gills or branchice or ctenidia. A pair of these hang down into 

 the mantle cavity on either side of the 

 foot (Fig. 176). 



Each gill is made up of two plates 

 or lamellae (Fig. 177, if) which lie side 

 by side and are united at the edges 

 except dorsally (Fig. 176). The cavity 

 between the lamellae is divided into* 

 J vertical water tubes by partitions called 

 interlamellar junctions (Fig. 177, ilj). 

 Each lamella consists of a large number 

 of gill filaments (if), each supported by 

 two chitinous rods (black spots in 

 Fig. 177, if), and covered with cilia. 

 FIG. 176. Diagrammatic Openings, called ostia, lie between the 

 11 filaments, and blood-vessels (v) are 

 i, right auricle; 2, epibran- present in the interlamellar junctions 



chial chamber; 3, ventricle; , ,-, 



4, vena cava; 5, non- and filaments. 



glandular part of kidney ; Water -is drawn through the ostia into 

 7' intestine'iiiioot ^ peri- tne water-tubes by the cilia which cover 

 cardium; 9, shell; 10, liga- the gill filaments; it flows dorsally into 



ment of shell. (From Shiplev , , . .j i i i z. /T" /c \ 



and MacBride, after Howes.') the Abranchial chamber (Fig. 176, 2); 

 from here it enters the dorsal mantle 



cavity and passes out through the dorsal siphon (Fig. 175, ex. sph). 

 The blood which circulates through the gills discharges carbon 

 dioxide into the water and takes oxygen from it. Respiration 

 also takes place through the surface of the mantle, 



EXCRETION. The organs of excretion are two U-shaped 

 kidneys or nephridia lying just beneath the pericardium, one on 

 either side of the vena cava (Fig. 175, kd). Each kidney con- 

 sists of a ventral glandular portion (kd) into which the pericar- 

 dium opens (r.p.a) by a ciliated slit and a dorsal thin-walled 

 bladder (bl) which opens to the exterior through the renal aperture 

 (r.ap) . Some excretory matter is probably driven into the kidney 



