700-660 million years ago


Although sponge cells do not organize to form tissues, some are contractile (which have been referred to as "almost muscles"), control over pH in digestion, phagocytes which can distinguish between self and nonself, and features of their gametes which are shared with higher animals.

Some sponges possess contractile myocytes similar to smooth muscle and these cells are associated with AChE, an enzyme important in the function of neuromuscular junctions in vertebrates (Harrison, Vol. 2). These contractile cells have been referred to as "almost muscles" (Mackie, 1990).

Controlling the acid levels of digestion is important since many digestive enzymes have an optimum pH at which they work best. The typical eukaryotic cell possesses intracellular organelles called lysosomes which digest molecules in an acidic medium. In many protists and sponges, the intracellular digestion which follows phagocytosis occurs first in an acidic environment, then in an alkaline environment, interestingly similar to the sequence in the stomach and intestine of vertebrates (Barrington, p. 172).

All animals possess amoeba-like cells (similar to white blood cells which perform phagocytosis) which float in the fluid around the body cells. Such phagocytes are even known in the most primitive animal groups, such as sponges and starfish, and are present in invertebrates which lack a true circulatory system (Hoar, 1983). These ameobocytes, like white blood cells, may be full of inclusions, some of which are phagosomes (Harrison, Vol. 2, p.47).

In sponges, sexual reproduction occurs without gonads-sex cells may originate from anywhere any part of the animal (Beklemishev, vol. 2). In sponges, ova move like amoeba and may perform phagocytosis (Barrington, p. 386). Spermiogenesis occurs to modify the shape of the male gamete, although no acrosome is formed on the sperm (Harrison, Vol. 2). Oocytes are large cells, about ten times the size of the oogonia from which they developed (Harrison, Vol. 2).