Explanation

The cavity common to all sponges is spongocoel or paragastric cavity. It is lined by endoderm, which contains a single layer of collared, flagellated cells, called choanocytes. Each cess contains a single nucleus, 1-2 contractile vacuoles, food vacuoles, blepharoplast, rhizoplast and a single basal granule (kinetosome) from which a single, long, whip-like flagellum is originated.