Introducing Chordata

A thumbnail sketch of basic anatomy and phylogeny:

Myllokunmingia fengjiaoa from Shimeld and Holland 2000
Vertebrate innovations Proceedings of the National Academy of Sciences
Based on current research, the ancestral vertebrate was a free-swimming suspension feeder like the early Cambrian Myllokunmingia fengjiaoa (right) described in 1999. Lets get acquainted with its body plan in its most basic outline.

Wahlert et al. 2013 Biology 1003, Survey of the Living World Baruch College
  • Feeding apparatus: Largest organ of body was the pharynx, a barrel shaped structure with mouth at one end and row of slits along the side, used to filter organic material from water.

  • Digestive system: This material is passed to a gut tube where it is digested. Ultimately solid waste is expelled through the anus on the underside of the body. Attached to this tube was a large organ, the liver, which converted food to easily stored forms, removed toxins from blood, and stored food.

    Gills: Wikipedia
  • Respiratory system: Gills, sheets of thin tissue rich in blood vessels suspended from the bars separating slits of pharynx allowed gasses (mostly O2 and CO2) to be exchanged between body and environment. Supporting structures of pharyngeal bars called branchial arches (= gill arches).

    Radinsky, Leonard, 1987 Evolution of Vertebrate Design
  • Excretory system: Other than CO2, wastes are filtered from the body by a pair of kidneys, located above and to either side of the gut.

  • Reproductive system: Just behind the kidneys are gonads (ovaries and/or testes), which produce reproductive cells.
  • Circulatory system: Gasses and nutrients carried to body tissues through blood, which passed through blood vessels. The circulatory system is closed. Blood is pumped by a muscular heart, traveling from heart to gill arches to body tissues in arteries exchanging gasses in tissues while travelling through capillaries, and returning to the heart through veins. Note, however, two special capillary beds:

    Two major arteries are the ventral aorta, which takes blood from heart to gill arches, and the dorsal aorta, which receives it from the gills and carries it down the length of the body.
    Musculo-skeletal system:

  • Nervous and endocrine systems:

    From Wikipedia
    Arguably the best living approximation of the ancestral vertebrate is the ammocoetes larva of the lamprey. The living creature grows to roughly 4 cm length before undergoing metamorphosis into adult parasitic lamprey. If we imagine the ammocoetes phase as the final life stage, we have something resembling the earliest fossil vertebrates.

    Vertebrata in context

    Animals in the broadest sense:

    The phylogeny of major animal groups appears at right. Included are:

    "Porifera," - the sponges, the simplest animals have distinct cell types but don't really have proper tissues and certainly not organs. One cell type is indistinguishable from choanoflagellates, the closest single-celled relatives of animals. Have amazing powers of regeneration. Organizationally, they are living sieves, filtering microscopic food particles from water.

  • Cnidaria and Ctenophora: represent the next simplest grade of organization, with distinct tissues (ectoderm and endoderm only) and organs. Still the gut is a simple pocket with an opening that can function as mouth or anus depending. Some examples:

    From Erectus
  • Bilateria: Animals with:
  • Among such animals, we see two major branches identified by distinct patterns of embryology. (Later)

    Protostomia briefly noted:

    Protostomia contains the vast majority of animal diversity, including many familiar animals. Among them are two major groups, Ecdysozoa and Spiralia.

  • There is a trend in these active animals toward the evolution of special sense organs (eyes, antennae) and toward concentrating them at the front end along with an expansion of the nervous system to deal with the information they provide (heads).
  • And yet, these structures originate independently. Consider some close relatives of the major groups: Tardigrades for ecdysozoans, and phoronids for Spiralians.



    Deuterostome survey: The deuterostomes contain Chordata (us), Echinodermata, and some minor groups. Of all of the major natural groups of animals, echinoderms are our closest relatives, and yet they are our weird relatives.