ABERRANT ANIMALS-THE MOLLUSKS AND ECHINODERMS 257 



/ 



pericardial cavi 



JD+cs+inc 



auricle 



bladder. 



typblosole 

 _ven+rick 



-VcoGCQva 



qonads 



cffererrj- qill 

 \)cssels 



in+cs+iDc 



bypobrancbial 

 cavity 



sbcll 



foof. 



.cpibroncbiat space 



kidney 

 _Dcrvc 



affcrenVqill 

 vessels 



iD+erlanTcllor 

 junction 



lor7>ellQe 

 qill 



Fig. 12-7. Cross-section of the clam through the region of the heart. 



adhesion, the pallial line is formed on the 

 shell. The heavy muscular foot lies directly 

 beneath the mantle and extends anteriorly 

 from the mid-portion of the body. Just 

 posterior to the foot and beneath the man- 

 tle are two pairs of gills. The dorsal portion 

 of the body, directly above the muscular 

 foot, contains the internal organs of the 

 animal and is called the visceral mass. 

 The four plate-like gills are attached 



from a point between the siphons to the 

 region just opposite the umbo ( Figs. 12-6, 

 12-7 ) . They hang freely beween the mantle 

 and the visceral mass. Each gill is made up 

 of two plates, the lamellae, which are held 

 together by bridges of tissue. The cavity 

 between the lamellae is divided into sepa- 

 rate water tubes. The lamellae are thrown 

 into vertical folds called gill bars and are 

 reinforced bv chitinous rods. In addition 



