210 



METAZOAN PHYLA 



together. A little way from the ventral margin and parallel to it is seen a 

 small groove, known as the pallial line, which marks the attachment of 

 the muscle layer of the mantle, or pallium. 



Ver?fr/'c/e. 



Vencf 



Suprabran^ 



ch/cr/ 



chambers 



Body 

 /ncrss 



3^e// 



/r7-/-es-/^/'r7e 



Per/carc^/a/ 

 coiv/'^y 



Aun'c/& 



HiWr;ey 



G///S 



/nfest/'ne 

 Marrrf/e 



Fig. 118. — Diagrammatic cross section of a fresh-water mussel. (From Parker and 

 Haswell, " Text-Book of Zoology," after Howes, by the courtesy of The Macmillan Company.) 



When viewed in cross section the shell is seen to exhibit several layers : 

 (1) a horny layer or periostracum, sometimes called the epidermis; (2) 



of ureter 



Per/carc//'a/ 

 cav/fy 



Verff-r/'c/e 



Ani-er/'or 

 Ofo/efucy-or 



/77tJSc/£' 



Posfen'or 

 adductor musc/e 



V/sceral gfangh'on 



Ana/ open/rrg 



C/oaca 



Dorsa/ 

 siphof^ 



Ventra/ 

 s/pho/7 



G/// 



Pe>c/cfJ^ 

 Cerebrop/eurar/ 



Fig. 119. — Diagram of the internal anatomy of a fresh-water mussel. {Compiled from 

 various sources.) The mantle and gills on the near side are not shown, and the body is indi- 

 cated as having had a part of the wall cut away. The stomach, liver, gonad, pericardium, 

 and kidney are shown in section. 



a series of layers of carbonate of lime, together known as the prismatic 

 layer; (3) a layer of 7iacre or mother-of-pearl, also carbonate of lime. 



