The Invertebrates: Animals without Backbones Help
Introduction to The Invertebrates: Animals without Backbones
All animals are labeled as heterotrophs, in that they must eat other organisms or organic matter to obtain energy for their survival. Also, all animals are eukaryotes, because their cells contain nuclei.
There are two broad types of animals – the vertebrates (animals with backbones) and the invertebrates (animals without backbones). We are discussing the invertebrates, first, because they are the older group and the first to appear in the Fossil Record.
There are several ways in which invertebrate animals can be subdivided into different main types or groups: for example, according to whether true body tissues are present or according to the overall pattern of body form.
Presence Or Lack Of True Body Tissues
Recall that a tissue is a collection of living cells, plus the intercellular material located in the spaces between them. Most invertebrates (and other animals) contain true body tissues: i.e., collections of similar cells that are specialized to perform only certain body functions. Epithelial tissues, for example, are collections of cells specialized for covering and lining body parts, while connective tissues help strap or link different body parts together.
This main group of animals are technically called the eumetazoans ( yew -met-uh- ZOH -ahns). The eumetazoans are literally the “animals” (- zoans ) that contain “true” ( eu -) tissues that are formed “after” ( meta -) repeated cell division in the embryo. Both a starfish and a lobster, for instance, contain specialized tissues playing particular roles in different parts of their bodies. Hardened epithelial tissue covers the tough, horny back of the lobster, whereas the soft muscle tissue (commonly eaten by humans) lies within its body and legs.
The other main group of animals are the parazoans ( pair -uh- ZOH -ahns). The parazoans are a small group of invertebrates that literally lie “beside” ( para -) most other “animals,” not quite belonging. The parazoans lack true tissues. This makes them basically different from the eumetazoan animals with tissues. The primary living examples of parazoa are the sponges (Figure 10.1). Sponges are members of the Phylum Porifera (poor- IF -erah), because their bodies are full of “pores” or holes. Sponges are usually stationary animals attached to the ocean floor. They consist of tube-like or wider vase-like colonies of cells having some specialization, but not enough to create individual tissues. Cells with flagella help sweep large quantities of seawater in through pores along the sides of the sponge. The flagellated cells are called collar cells . As the nutrient-rich seawater is swept in, food particles are caught within the sticky mucus in the oval collar of tiny branches located at the base of the flagellum. The particles are then ingested by phagocytosis. Neighboring amoeba-like cells help with the digestive process. Although the collar cells and amoeba-like cells are different, they still do not form separate types of tissues.