REFERENCE LETTERS. 



a. Auricle. 



aa. Anterior aorta. 



aam. Anteroir adductor muscle. 



ac. Apical cilia. 



ap. Apical plate. 



apa. Anterior pallial artery. 



apn. Anterior pallial nerve. 



aps. Anterior pallial scar. 



ar. Archenteron. 



ba. Branchial artery. 



ba'. Branches of the branchial artery. 



ba". Branches of the branchial artery in the modified filaments. 



bg. Byssal gland. 



bn. Branchial nerve. 



bs. Blood space. 



bv. Branchial vein. 



bv'. Branches of the branchial vein. 



c. Cartilage. 



cc. Cerebral commissure. 



eg. - Cerebral gangloin. 



cgl. Cuticular gland. 



CO. Cornea. 



cp. Cartilage pit. 



cpc. Cerebro-pedal connective. 



cpm. Circular pallial muscles. 



cpm'. Circular pallial muscles of the infolded ridge. 



cpn. Circum-pallial nerve. 



or. Chitinous rods. 



cvc. Cerebro-visceral connective. 



e. Excretory organ. 



f. Foot. 



fa. Foot artery. 



fc Feeding cilia. 



fe. Free edge of the unattached lamella. 



fm. Foot muscle. 



fn. Foot nerve. 



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