A class of the phylum Porifera, including sponges with a skeleton of one- to four-rayed siliceous spicules or of spongin fibers or both. Several genera lack a skeleton, and it is through a study of these seemingly primitive forms that the complicated structure of most adult Demospongiae may be understood. The Demospongiae constitute the most abundant and widely distributed group of sponges, occurring in the sea from the tidal zone down to abyssal depths (at least to 18,000 ft or 5500 m). Three families have invaded fresh water. The species vary in size from thin encrustations several centimeters in diameter to huge cake-shaped forms which may measure up to as much as 6.6 ft (2 m) in diameter.