Animal body plans

(A) Formation of Coelom - Protostomes vs Deuterostomes

In echinoderms and chordates, for example, the anus forms first, and then the mouth. In animals belonging to several other important phyla, on the other hand, such as arthropods, annelids and molluscs, the mouth forms first, followed by the anus. For this reason arthropods, annelids and molluscs are sometimes called protostomes (Greek: proto = the first; stoma = mouth) and echinoderms and chordates are called deuterostomes (Greek: deutero = the second; stoma = mouth). It is also possible to take an most echinoderm or chordate embryos at the 2-cell or 4-cell stage of development, separate the cells, and still have each cell continue on to develop into a complete, viable organism. This is not possible with protostome embryos. If they are separated at this stage, the cells will not develop into complete, viable organisms. For this reason, protostomes are said to display determinate cleavage, and deuterostomes are said to have indeterminate cleavage. You will be introduced in lecture and in subsequent laboratory exercises to other important developmental characteristics that distinguish protostomes from deuterostomes.  Figure 32.7 of the textbook illustrates some differences between protostomes and deuterostomes.

(B) Symmetry

Animals show different patterns of body symmetry. Some groups, such as the phylum Porifera, show no particular pattern of symmetry (asymmetry). That is, no line of bisection exists that could divide the organism into similar-looking halves. Other groups, including the Cnidaria and Echinodermata show radial symmetry, where more than one hypothetical bisection can be visualized (lines A, B, and C can each bisect the organism). A third pattern, seen in most phyla of animals, is bilateral symmetry, where only one hypothetical bisection can be visualized (only line A can bisect the organism into two halves). While these patterns can be seen readily, the implications of the different patterns of symmetry are important. Make a list of advantages as well as constraints of each of these three patterns of symmetry. In later labs where you are examining different animals, test your ideas in your list to see if you were on the right track.

(C) Body Cavities

One of the primary ways zoologists group animals has to do with the presence or absence of a coelom, and how the coelom is formed. A coelom (Greek:  coel = hollow; pronounced “see-lome”) is a fluid-filled cavity between the alimentary canal and the body wall.  The peritoneal cavity in our abdomen is one part of our coelom, and there are similar spaces around our heart and lungs.  However, the type of coelom (or even its existence) differs among groups of animals – both in its structure (such as what types of tissues surround it) and its mode of development.  There are three structural types of body plans related to the coelom, as illustrated in Figure 32.6 of the textbook.  These are

1.  Acoelomate, in which no coelomic cavity exists.  Find an animal in your textbook or in the lab that is an example of this body plan.

2.  Pseudocoelomate, in which a coelom exists, but it is lined by mesoderm only on the body wall, not around the gut.  What is an example of a pseudoceoleomate animal?

3.  Coelomate (or Eucoelmate, or “True” Coelom), in which the coelom is lined both on the inside of the body wall and around the gut by mesoderm.  Animals with a true coelom also have mesenteries, which suspend the body organs within the coelom.
Body Plan
Tissue Layers
and Cavities

There are two types of “true” coelomic cavities.  These do not differ in structure, but they do develop in different ways.  In most protostome animals with a true coelom, the body cavity originates as a split within a bud of mesodermal tissue at the time of gastrulation.  This method of coelom formation is termed schizocoelous (Greek:  schizo = split), and occurs in animals like segmented worms and mollusks.  In most deuterostome animals the coelom originates from an outpocketing of the archenterons during gastrulation.  This method of coelom formation is called enterocoelous.  You can see this outpocketing on some of the slides of sea star development.  These methods of coelom formation are illustrated in Figure 32.7 of the textbook.

(D) Segmentation, Cephalization, and Tagmosis

Segmentation, also known as metamerization, is the structural grouping of parts of an animal body into discrete segments. Cephalization means that there is a head, and therefore a concentration of sensory organs, feeding organs, and centers of neural integration near the anterior end of the animal. While at first seeming a bit simplistic, cephalization has tremendous implications for animals. Can you think of any advantages of cephalization? Is cephalization dependent on any particular type of symmetry? Can cephalization occur in protostomes as well as deuterostomes? Tagmosis occurs in segmented animals where groups of segments are organized into functional units. A good example is in arthropods, where segments are grouped into body regions like the head, thorax, and abdomen, each having its own suite of functions.

(E) The Evolution of Body Plan in Major Animal Phyla

The features of body plans we have introduced here will be discussed at greater length in subsequent exercises.  Features of these body plans are important aspects of animal evolution, producing different “body plans” that influence the size, locomotory abilities, and feeding and digestive capabilities of animals.  Figure 32.4 of the textbook shows an overview of the different features of body plans in animals.  Use this overview as you study the evolution of animals during the coming weeks.  However, be aware that this diagram may not necessarily reflect the true evolutionary pathways of animals.  Figure 32.8 of the textbook shows one alternative hypothesis for animal evolution.  You will learn more about both of these hypotheses in coming labs and lectures.