The body of Eunapius calcuttanus is bulbous and elongated, featuring a smooth surface. Numerous flat and small oscula are present, forming distinct radial canals. The consistency of the sponge is soft and fragile. The megascleres in Eunapius calcuttanus are fusiform and slender, with smooth spines at both ends, forming lanceolate-shaped apical projections. Their length and width range from 0.18-0.22 mm and 9-15 µm, respectively. Gemmoscleres are slender, cylindrical, and curved amphioxea with small spines covering the entire body length. Their length and width range from 0.085-0.120 mm and 0.003-0.006 mm, respectively. The gemmules in this species are numerous, small, and spherical, exhibiting a polygonal shape with large air spaces in a well-developed pneumatic layer. Gemmoscleres are embedded in the pneumatic layer tangentially. The foramen is tubular, outwardly open, with a diameter ranging from 0.220-0.250 mm. The coloration of the sponge varies from light brown to light grey.