here's a quote stolen from Pocket Expert Marine Invertibrates pg 21
"Sponges are classified in the Phylum Porifera, a group where specific identifications of al but a few forms are impossible without examination of the internal skeletal structures. Some sponges have a defined shape and adult size, and these may often be identified from photographs. Most sponges, however tend to grow without a defined shape or size, making them hard to describe unambiguously."