Practice question

Molluscs belong to grade Triploblastica, tissues deriving from three germ layers: ectoderm forming epidermis and nervous system, mesoderm giving rise to musculature, coelom, heart, nephridia and gonads, and endoderm lining midgut and digestive gland. Development protostomous with spiral holoblastic cleavage, blastopore forming mouth. They are not diploblastic like Cnidaria and Ctenophora possessing merely two layers, nor acoelomate. True coelom reduced to pericardial, gonadal and renal cavities, yet triploblastic coelomate organization reflects advanced organ system level with centralized circulation and complex organogenesis distinguishing molluscs from lower metazoans and diploblasts.