Question

Describe how animal bodies (from sponges through the lophotrochozoans) have increased in complexity through the course of animal evolution. Your answer should include examples from several different lineages.

Answer 1

The animals are multicellular eukaryotes that often termed as metazoans bearing body with highly organized tissue or organ system and digestive tract with a specialized cellular network. During the evolutionary process, these lower eukaryotes bearing simplified cellular organization gradually developed into highly complex modern-day multicellular animals. If we think of these metazoans, they can be classified into two categories or lineages: Eumetazoa (eumetazoans) bearing specialized tissue system and Parazoa, without any tissue system. The lineage Parazoa involves phylum Porifera (e.g. Sponges) that further classified into three distinct groups such as Hexactinellida, Demospongia, and Calcarea. Another lineage called Eumetazoa was classified into two sub-lineages based on cleavage and symmetry: Bilateria and Radiata. The bilateral symmetry and triploblastic cleavage are the characteristic features of Bilateria while radial symmetry and diploblastic cleavage are the features of group Radiata. The group Radiata was further classified into phyla Cnidaria (found in freshwater and marine, eg. sea anemones, corals) and Ctenophora (possess sticky cells called colloblasts, e.g. The Comb Jelly).

The group Bilateria was classified further on the basis of the presence or absence of a body cavity (Coelomates/Eucocoelomates or Acoelomates). Acoelomates or acoela are devoid of the body cavity. The best example of this group is flatworm without any cavity. The Coelomates possess a true body cavity or a fluid-filled body cavity present between the body wall and digestive tract. They were further classified into Protostomia and Dueterostomia based on the fate of the blastopore. The Protostomia are the coelomate invertebrates that bear the first mouth, a result of the blastopore. The Dueterostomia is characterized by the appearance of the anal opening as a result of the blastopore. The Dueterostomia were further subdivided into Chordata and Echinodermata based on symmetry and skeleton. The Chordata possesses bilateral symmetry and their internal skeleton is composed of bones and cartilages. The Echinodermata are marine invertebrates often featured by the presence of radial symmetry and a mesodermal skeleton made up of ossicles. The Protostomia were classified into two sub-categories: Ecdysozoa and Lophotrochozoa. The members belonging to Ecdysozoa possess an ability to shade their exoskeleton several times throughout their life. The Ecdysozoa was categorized into Arthropoda and Nematoda. Arthropods are nothing but invertebrate animals bearing segmented body, paired jointed appendages, and exoskeleton and are characterized by members such as insects, crabs, spiders, mites, etc. The Nematoda group members are characterized by bilateral symmetry, worm-like organisms possessing cuticle, a non-cellular layer. They include two sub-categories eg. Phasmidia and Secernentea. The roundworms are the best examples of nematodes.

The group Lophotrochozoa often divided into several sub-categories involving Platyhelminthes (e.g. flatworms), Rotifera (e.g. rotifers), Ectoprocta (e.g.), Brachiopoda, Annelida (e.g. annelids), and Mollusca (e.g. mollusks). The Lophotrotrochozoa group members possess trochophore larvae and lophophore. The Lophotrochozoa group members are known by their asexual mode of reproduction. The Platyhelminthes don't possess specialized circulatory or respiratory organs but possess a simple digestive cavity. They have bilateral symmetry and are triploblastic in nature. The phylum Rotifera are also called as wheeled animals and include Brachionus plicatilis, Lacane bulla, etc. The Ectoprocta are often marine and sometimes freshwater animals. They are solitary in nature. They possess bilateral symmetry, un-segmented body pattern, and V-shaped digestive tract. The examples include Victorella, Clavopora, etc. The Brachiopoda are often termed as pedicle sheath. The example includes Magellanea. The Annelida possess segmented body pattern and bilateral symmetry. The examples include earthworm, leech, lugworm, etc. The Mollusca possess bilateral symmetry, although some members lost such a pattern. Examples include cuttlefish, squid, octopus, etc. In this way, one can summarize that animal evolution progressed through several transitions which involve symmetry, body pattern, the fate of all dermal layers, etc.