Biology · Kingdom Animalia. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Coelomates have a true body cavity lined by mesoderm. In Annelida, Arthropoda and Mollusca the mouth forms first in the embryo (protostomes). In Echinodermata, Hemichordata and Chordata the anus forms first (deuterostomes), which is why spiny, radial starfish are placed close to chordates. Their larvae are bilateral.
Learn each phylum by its special features: segments and setae in annelids, jointed legs and a chitin exoskeleton in arthropods, a mantle and muscular foot in molluscs, tube feet in echinoderms.
Every chordate has, at some stage of life, a notochord, a dorsal hollow nerve cord and pharyngeal gill slits. In vertebrates the notochord is replaced by a vertebral column.
Students give the chordate nerve cord as ventral and solid. That describes annelids and arthropods. In chordates it is dorsal and hollow, lying above the notochord.
Answer: Phylum Mollusca, class Pelecypoda (Bivalvia).
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