Animal Tissue and Organ Systems in Lower and Higher Phyla

Sponges are multicellular animals with cellular level of organization.
Cells do not form tissues and are less specialized.
Sponges are sessile, aquatic, mostly marine, and few species are freshwater.
Body is asymmetrical or radially symmetrical.
Sponges have diverse forms, colors, and sizes, ranging from a few mm to 2 m in diameter.
Body is perforated by numerous pores (ostia and osculum), canals, and chambers through which water continuously flows.
There is a single cavity inside the body, called paragastric cavity (spongocoel), lined by choanocytes.
Internal skeleton is composed of calcareous or siliceous spicules and/or spongin fibers.
Digestion is partly extracellular and partly intracellular.
Respiration and excretion occur through simple diffusion through the body surface.
Simple diffused nervous tissue is present in the form of a nerve net.
Sensory cells are present with sensory organs in medusa.
Some colonial forms secrete massive external calcareous skeleton.

Multicellular animals with tissue level of organization.
Body is built up of two layers: outer ectoderm and inner endoderm.
Bilateral symmetrical animals, except adults of most echinoderms.
Tissue formation is primitive in lower phyla and more developed in higher groups.

Diploblastica: Phylum Cnidaria (Coelentrata).
Triploblastica: Phyla with a middle layer, or mesoderm, formed in between the ectoderm and endoderm of embryo.

Coelomata: Extensive fluid-filled cavity (coelom) completely lined with mesoderm, separating the digestive tract from the outer body wall.
Pseudocoelomata: Fluid-filled body cavity not completely lined with mesodermal tissue.
Acoelomata: No internal body cavity, or coelom, in the mesoderm between the digestive tract and the outer body wall.