The external morphology of the shell, anatomy and hinge of the right and left valves of P. casertanum from the Vashutkiny Lakes. A – external morphology of the right valve; B – internal anatomy of the dissected P. casertanum, C – teeth of the right valve; D – teeth of the left valve; E – general view of the male and female gonad of P. casertanum: o – oocyte, n – nucleus, sp – spermatocytes and spermatids aggregations