Simple Animals

Sponge

Almost all animal cells are organized into tissues. Sponges are animals of the phylum Porifera. A sponge's body is hollow and is held in shape by the mesohyl, a jelly-like substance made mainly of collagen and reinforced by a dense network of fibers also made of collagen. The inner surface is covered with choanocytes, cells with cylindrical or conical collars surrounding one flagellum per choanocyte. Sponges are freaking simple. So simple that they can't even move. Sponges just filter water for food just like bacteria. Sponges are classified as animals because they are multicellular, heterotrophic, and lack cell walls. Sponges are the simplest animals. They are classified as animals because they are multicellular, heterotrophic, and lack cell walls. They lack true tissues and organs, and have internal skeletons of spongin and/ or spicules of calcium carbonate or silica. Sponges have no body symmetry. The shapes of the bodies of sponges are adapted for maximal efficiency of water flow. Water enters through the central cavity, deposits nutrients, and leaves through a hole called the osculum. All sponges are sessile aquatic animals. Although there are freshwater species, the great majority are marine (salt water) species, ranging from tidal zones to depths exceeding 8,800 m (5.5 mi). Jellyfish are animals of the phylum Cnidaria.Jellyfish are found in every ocean, from the surface to the deep sea. A few jellyfish inhabit 
freshwater. Large, often colorful, jellyfish are common in coastal zones worldwide. Jellyfish have.
roamed the seas for at least 500 million years,and possibly 700 million years or more, making them 
the oldest multi-organ animal. Cephalopods are really smart. Cephalopods became dominant during the Ordovician period, represented by primitive nautiloids. Octopi and Squids are Cephalopods. If you just do a youtube search on Octopi you will see all kinds of videos of them opening jars stealing people's video cameras. Diploblasty is a condition of the blastula in which there are two primary germ layers: the ectoderm and endoderm.Diploblastic organisms are organisms which develop from such a blastula, and include cnidaria and ctenophora, formerly grouped together in the phylum Coelenterata, but later understanding of their differences resulted in them being placed in separate phyla.

The endoderm allows them to develop true tissue. This includes tissue associated with the gut and associated glands. The ectoderm on the other hand gives rise to the epidermis, the nervous tissue, and if present, nephridia.

Simpler animals, such as sea sponges, have one germ layer and lack true tissue organisation.

All the more complex animals (from flat worms to humans) are triploblastic with three germ layers (a mesoderm as well as ectoderm and endoderm). The mesoderm allows them to develop true organs.

Jellyfish have no brain nor central nervous system, but employ a loose network of nerves, located in the epidermis, which is called a "nerve net".