The first dichotomous branch point in the phylogenetic tree of animals distinguishes between the parazoans and the eumetazoans; organisms lacking true tissues versus those that have truly specialized tissues. The first phylum we will discuss are the parazoans of Phylum Porifera, the sponges.
Some scientists still question whether sponges are really animals and, if so, are they individuals or colonies of individuals? While sponges are composed of a loose collection of cells, they lack the true tissue-level organization that is characteristic of eumetazoans. There is evidence that supports both choices.
Sponges are asymmetrical; they do not have a plane that divides them into mirror images. They also do not have true Hox-genes, but do have Hox-like genes that may be involved in body pattern formation.
Sponges have an epidermis composed of tightly packed cells, underneath which lies a gelatinous matrix and a few specialized cell types that surround a central cavity termed the spongocoel. The spongocoel is connected to the outside via an opening called the osculum. The spongocoel is lined with choanocytes, which are cells that have a central flagellum and a sticky collar that surrounds the flagellum. Which organisms do these cells look like? Water is drawn into the spongocoel through the pores, and food particles in the water may pass the sponge's choanocytes. The flagellum of a choanocyte pulls in food particles, which get stuck in the sticky mucus of the collar and are picked up by amoebocytes. Amoebocytes are mobile and can transport nutrients throughout the body of the sponge. Water then exits the sponge through the osculum.
A sponge has an embryonic form similar to a blastula because it is hollow. One-half of this embryonic form is flagellated, hence the embryo is free-swimming. This hollow, half-flagellated ball will eventually settle and become stuck to a substrate. The flagellated half will invert, and the point where it inverts will become the osculum. The space created during inversion will become the spongocoel. Note that the adult sponge is sessile, whereas the embryonic form is motile.
While we mainly use synthetic sponges, natural sponges are still sold for household and artistic use. The photo above shows how sponges are sold at a market in Crete, Greece.