214 



METAZOAN PHYLA 



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

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

 the muscle layer of the mantle, or pallium. 



Veryfr/c/e. 



Vencf 



Suprabran^ 



ch/cr/ 

 chambers 



Body 

 mass 



3/7e// 



/rrZ-esf/'ne 



Per/cara//'a/ 

 caW-/-y 



Aur/'c/e 



H/Wf7ey 



G/7/s 

 fnfesf/'ne 



Fig. 127. — 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) 



Esophagus 



Opening 

 o-f ure-her 



Per/carc//al 

 cav/fy 



An+er/on 



crc/ci/ucto/' 



/77usc/e 



Ve-nfric/e 

 Pcsfenor refmcfor muscle 



Posfen'or 

 ac/ductor muscle 



Visceral gang //on 



Anal opening 



C/oaca 



Dorsaf 

 siphon 



Ventral 

 siphon 



Labial palp 



G/J/ 



\ 

 Foof Peolal 

 gang//on 



Cerebrop/earcr/ 

 ^angZ/or? 



Fig. 128. — 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 nacre or mother-of-pearl, also carbonate of lime, 



