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The phylum Porifera is a diverse group of primitive animals commonly referred to as sponges. Until the early 1800s, sponges were actually classified as plants. Sponges occur in fossil records back to the Precambrian Era (over 600 million years ago) and were the most important contributors to reefs during the Palaeozoic and Mesozoic Eras (Hooper and Van Soest 2002). All members lack defined organs; however, differentiated cells within connective tissue perform necessary biologic functions. Unique systems of water canals facilitate transport of food, waste products, and gametes. Most of the 5000‐plus species are marine with just 3% of sponges occurring in freshwater environments. Sponges are normally found on firm substrates in shallow water, although some occur on soft bottoms.

Sponges maintain a close association with a variety of bacterial genera. While some can be pathogenic, most form a symbiotic relationship with the sponge and provide nutrients. These symbionts are what impart the vast array of bright colors seen in some sponges.

Surgery on sponges is frequently employed to study regeneration and reproductive strategies (Korotkova 1970; Simpson 1984; Kuhns et al. 1997; Henry and Hart 2005). A recent study by Borisenko et al. (2015) examined regeneration in the sponge Halisarca dujardini (Figures 4.1 and 4.2). The authors were interested in healing at the cellular level and use microscopy to describe the production of a blastema, an accumulation of undifferentiated cells that will become a new tissue or organ (Carlson 2007). In the case of sponges, the blastema differentiates into new sponge tissue capable of nearly all biologic functions. In the Borisenko et al. (2015) study, the authors used Castroviejo scissors and microscalpels to perform the tissue excisions under a stereomicroscope.