Phylum Chordata

One of the last phylum’s I will cover at this time is that of the chordates.  I chose to leave this one for the end of the semester due to its significance to us as humans, in that we are vertebrate Chordates!  Often a subject of confusion surrounding Chordates lies within their diagnostic aspects.  To elaborate, we have heard time and again (as well as thought it myself) that Chordates are exclusively vertebrates.  Though this is true for many of the more well-known classes of Chordata, this is not a universally accurate statement. 

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Common Vertebrate Chordates. Photo by Herbert W Rand, 1950 (CC 0 Public Domain)

Rather the trait this schema of thought refers to is that of the Notochord that designates Chordates from the rest of the Animalia kingdom.  The Notochord is a malleable rod running the length of the organism’s body, to which the rest of the skeletal structure relies upon for foundational support.  There are three other defining traits to Chordates: the presence of a tail extending past the anus, a hollow, dorsal nerve cord (think spinal cord in humans!) and pharyngeal gill slits, with the ability to be modified for specialized functions in mature vertebrates [3].  Though this might sound outlandish, all human fetuses are actually developed, starting with gills!

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9-Week Human Embryo. Photo by Ed Uthman, 2000 (CC by 2.0)

            There is a myriad of classes that comprise phylum Chordata.  These classifications are constantly changing along with taxonomic research and continued development of our understanding of life on Earth.  Let’s begin with some of the invertebrates. 

           Subphylum Urochordata is made up of tunicates, otherwise deemed ‘sea squirts’.  They are exclusively found in marine environments and seem strange to loop into the same category as vertebrates.  Even more interesting is their manner of nutrient collection and waste expulsion.  What is really a rather simple system, there is an incoming siphon, that draws in water and food particulates that may be floating in this medium.  These nutrient particulates are then passed down to the intestine where they are processed for sustenance.  The excess water and waste products are rid through the other siphon known as the excurrent siphon.  Even more surprising is the fact that these animals only show all traits of Chordates (much like humans) when they are in larval stage [3].  Urochordata may show themselves at a mature stage in a colonial form as seen below.

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Komodo Tunicate. Photo by Nick Hobgood, 2006 (CC by 3.0)

           Subphylum Cephalochordata exhibit all the traits of Chordata as lancelet adults.  Like Urochordata, they are also marine organisms and may be found world wide in shallow waters.  Often they may be observed at the benthic level, where they burrow themselves into the sediment but leave their backsides exposed as a foraging mechanism [2].  The anterior portion of these organisms resembles that of the face of shrimp or of a praying mantis.  Their manner of feeding is through that of filtration of the nutrient rich waters around them.  Cephalochordata have been described as ‘fishlike’ in comparison to their Urochordata counterparts.  The species that comprise this subphylum are relatively small ranging from 5-15 cm in length.  And though they express a closed circulatory system, they have no heart.  Rather their blood is oxygenated via their gill slits and recycled throughout.  Their dorsal nerve runs throughout their body, however at the anterior end does not form a brain complex [2].

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Branchiostoma lanceolatum. Photo by Hans Hillewaert, 1997 (CC by 4.0)

           Moving onto the vertebrate category, it is important to start off with the superclass Agnatha, more commonly known as the jawless fish.  Two of the over-arching classes comprising Agnatha are Cephalaspidomorphi and Myxini.  It is commonly accepted that in the development of higher vertebrate-beings evolutionarily began with the segmentation of a vertebral column, that would then give rise to a ‘backbone’ [5].  This vertebral column typically incorporates and/or replaces the primitive notochord.  These adaptations parallel the development of sensory organs and a complex neural systems, brains encased in a skull!  Vertebrate Chordates also pertain to the trait of exhibiting bilateral symmetry having a closed circulatory system with a chambered heart.  The degree to which the heart is chambered (i.e. one, two, three and four chambered hearts), varies with taxonomic class.

The Cephalaspidomorphi (lampreys) at a juvenile stage are filter feeders.  Living off of the nutrient particulates floating in fresh water.  The latter being important to the development of lampreys in the juveniles wait till maturity to migrate into salt-marine environments.  In the ocean, lampreys may often be observed attached to larger reef fish and megafauna.   They press their mouth to their host and using a tongue, similar to that of a rasp as was seen in gastropods in Phylum Mollusca, drawing blood out of their victim.  Such a relationship can only be described as parasitic as this usually does not result in the death of the host. 

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Sea Lamprey. Photo by Joanna Gilkeson, 2015 (CC by 2.0)

           The Myxini Class are ecologically called hagfish and reside solely in marine environments.  They are similar in structure to eels and as previously stated, they are jawless.  Myxini can either be consumers or decomposers, feeding on the flesh of weakened or already dead fish.  They have also been known to prey upon small invertebrates.  Just like Cephalaspidomorphi, Myxini tongues resemble a rasp, and are similar to serrations on a knife.  One of the defense mechanisms exhibited by Class Myxini resembles that of many amphibians (Foreshadowing), in that when threatened they will release mass amounts of high-viscosity fluids, to distract, escape from, confuse or deter potential predators [5].

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Eptatretus soutii. Photo by Stan Shebs, 2005 (CC by 3.0)

           One might read the terms Osteichthyes and Chondrichthyes and deduce that their prefixes correspond with the classes defining traits and they would be right.  Osteichthyes as their name suggests are boney fish, dwelling in both fresh and salt waters around the globe.  Boney fish are comprised of a hard skeleton and coated in slippery, sometimes sharp, scales.  An important feature of boney fish is that of their lateral line, which is exactly what it sounds like.  It is an ambiguous vector that runs horizontally along the body of the fish, that is predominantly used in the detection of vibrations.  The lateral line has been attributed to the coordinated and navigational success of schools of fish, in which mass quantities of individuals conglomerate for the general benefit of the species.  There are currently over 34,000 known species of fish on the planet and that number is both growing and shrinking [3].  The paradox rests in human-kinds lack of knowledge of the seas, in addition to egregious overfishing practices, which has left much of the species abundance of various ecosystems to a saddening downward spiral. 

1. Oncorhynchus mykiss 2. Pontinus nematophthalmus 3. Aracana aurita 4. Peristedion gracile. Photo Public Domain (CC 0)

           For Chondrichthyes, the defining difference between this Class and their boney fish counterparts is the composition of their anatomical structural system.  This is in that Chondrichthyes are comprised of cartilage rather than bone connective tissues.  Cartilaginous fish are primarily comprised of Rays, Sharks and Chimaeras.  The vast majority of sharks are predators, some being apex in their food chain.  Much of their power and deadliness comes from their evolutionary adaptations to their physiology. Their streamlined and highly muscular body lends to their high proficiency as consumers [5].  The flattened bodies of rays contributes to their free-flowing nature through water as a medium.  Often times rays consume invertebrates that are found on the benthic level of the ocean.  However rays are highly diverse in size, phenotypic expressions and behavior.  Giant Manta Rays may be upwards of 7m in width and filter feed on masses of zooplankton, where the southern stingray may only be a few dozen centimeters in width, bottom feeding in coral reefs [4]. 

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Hypanus americanus. Photo by Barry Peters, 1992 (CC by 2.0)

Chimaera’s are interesting organisms, atypical in physiological nature and a cross of several categories as their name suggests.  Rather than a jawline filled with teeth, as the previously studied organism, Chimaera’s have a flat dental plate. This corresponds to a skull structure that then formulates the organisms jaw and mouth, otherwise unobserved in nature.  Origins of Chimaeroid marine species can be traced back upwards to 280 million years, predating the earliest dinosaurs of the Triassic period.

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Dunkleosteus Skull. Photo by Zachi Evenor, curtousy of Vienna Natural History Museum, Austria, 2014 (CC by 3.0)

  A January 2017 discovery of a fossilized Dwykaselachus oosthuizeni skull showed that there are few structural differences in ancient Chimaeras as to modern ones today.  CT scans of the fossil showed significant cranial nerves, inner-ear structure and nostrils, which are all exhibited by modern Chimaeras [9].  An incredible aspect of this would pertain to Earth’s projected geological history.  The presence of modern-day Chimaeras with little distinguishedness from ancient species means that these organisms survived two mass extinction events, lending support to the perseverant nature of these beings that dwell in the ocean depths.

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Modern Deep Sea Chimaera. Photo courtesy of NOAA, public domain (CC 0)

           Moving onto Class Amphibia seems appropriate as we make our way out of the waters and onto terrestrial environments.  It is well demonstrated through overwhelming evidence that this is how terrestrial life on Earth originated, from the seas [10]!  The definition of amphibious, as stated by Merriam-Webster, is anything that is related to or adapted for water and land.  Frogs, Salamanders, Toads and limb-lacking caecilians are perfect examples of this, hence their categorizations into the class Amphibia.  Morphological characteristics include smooth, moist epidermis, that is also porous to allow for the exchanges of gas, as Amphibians breath through both their skin and lungs [5].  There are cases of Amphibiae in which upwards to 25% or more of their bodily oxygen needs are fulfilled through dermal-respiration.  There are rare cases of amphibians that do not have lungs and exclusively breath through their skin [7].  Amphibians are susceptible to dry environments due to the necessity for their skin moisture; an overly arid setting could kill them!  That’s why one might often find them close to water, ponds, swamps, etc. not only for breeding grounds but also at their permanent residence.

amphibia, frog, amphibian, leaves, forest, American toad, frog
American Forest Frog. Public Domain, 2016 (CC 0)

           It is only appropriate as we move out of the waters of the salinized oceans and fresh water bodies to look to reptiles and their diversity as marine, amphibious and terrestrial beings.   The origin of Class Reptilia goes far back to the dawn of the dinosaurs in the Mesozoic era; following their extinction, the class managed to persevere and return with a vengeance.  Crocodiles, Alligators, Lizards, Turtles, Snakes and Basilisks are all inclusive of modern Reptilians.  The common notion of reptilians is that they are cold blooded, which is absolutely true; this means that they are ectotherms, living off of the kinetic energy that they absorb from the Sun and the environment.  The mechanism of how this behavior and interaction with abiotic factors may however be slightly more obscured than the obligate relationship itself.  Reptilians exhibit water-resistant scales at the surface of their skin, this serves to act as both solar panels and a source limiting water-loss.  Should these organisms get too hot while they bask in the sun, they simply seek shelter and shade. The vast majority of Reptiles lay amniotic, shelled eggs following an instance of sexual reproduction [5]. 

Reptiliae. Photo by Andreas Plank, 2010 (CC by 3.0)

Moving onto some of the more appealing organisms of this list, Class Aves comprises, you guessed it, Avians!  Pertaining to 28 Orders, 163 families, 1,975 genus’ and close to 10,000 species, birds are of the closet living relatives to the long-lost dinosaurs of the Jurassic period [1].  Birds are highly adaptive beings and were the basis for many touchstone studies, including the very famous case supporting survival of the fittest and adaptive advantage in Darwin’s finches (If you reader, are not familiar with this, it is a must, see the link!).  Aves have several adaptations for flight that vary within specific taxonomic class, but generally include, wings with feathers (for broadening surface area at minimal cost to weight), honeycomb-structured light-weight bones, as well as extremely advanced/sensitive nervous systems, incredible hearing and sharp vision.  Another incredible aspect of birds is their energy expenditure and endothermic nature.  Most species of birds function at an extremely high metabolic rate.  During reproductive stages, birds lay shelled eggs, incubated externally, that may be limited to a single egg or many [5].  When they are not flying or mating, they may typically be found foraging, hunting or eating to maintain homeostasis.  In the Turks and Caicos, where the KSC Tropical Marine Biology research trip takes place, we will have the pleasure of seeing a wide array of beautiful and intriguing birds.  Terrain variants and a multitude of islands provide a plethora of species to observe, including the Antillean Nighthawk, Brown Pelicans, Great Egrets, American Oystercatchers and Mangrove Cuckoos [6]!  Much more on general Ornithology may be found here:

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Tropical Bird. Photo by Martin Pettitt, 2013 (CC by 2.0)

            The last Class that will be covered in this post is arguably the most aesthetically appealing (cutest) and certainly most closely related to us as humans, as we are also of this Class!  Class Mammalia has many distinctive characteristics, with two of the most apparent being mammary gland in females, used for the rearing of young and the presence of fur/hair for insulation.  Similarly to Avians, Mammals exhibit a high metabolic rate as endotherms and often need to eat continually to survive, whereas an organism such as a Crocodile may be able to survive on one large meal per month [5].  For my highly perceptive individuals that might be reading this, the difference between hair and fur is pretty much semantic.  There is not chemical difference between the two; both being comprised of Keratin proteins.  There are a few miniscule notable differences in that hair is said to grow independently being long and soft, while fur grows in synchronous and tends to be on the shorter side and more coarse.  Much of this is purely based on specific organism [8].  The most important difference between Mammals and other Chordates is their large brain size in comparison to other vertebrates and their relative physiological size.  The cognitive abilities of mammals is without question and may be witnessed in Primates, Marsupials, Whales, Elephants, Bats and more.  Cognition, compounded upon a significantly heightened affinity for empathy sets us as Mammals apart from the rest of the natural world. 

Coat Baboons, Ape, Primates, Animals, Zoo, Zoo Animal
Baboons grooming eachother. Photo by Alexas Fotos, 2018 (CC by 2.0)

Keep in mind there is so much more to know about this Phylum and the rest that I have reported on in my website.  The purpose of these posts is to provide a broad overview and spark an interest in whatever sect of life one might prefer.  I strongly encourage you reader, to keep doing your own research and expound upon the information I have given you.  Perhaps one day, it will be you that is teaching me!


  1. “Aves.” (2019). Aves. Retrieved from
  2. Dr. Houseman, Jon G. Department of Biology at the University of Ottawa, 2019
  3., fishbase consortium
  4. Noaa. 2019. Fish & Sharks. Retrieved from
  5. Peachey, Donna & Gordon, The Biocam Museum of Life Series.  Kelowna, B.C. Canada VIY 7N8 Box 417 PBC, 2000
  6. Providenciales Birdwatching. (2019). Retrieved from
  7. Rockney, H., & Wu, K. (2016, April 08). All about amphibians. Retrieved from
  8. Scientific American, 2019. What is the difference between hair and fur?  Retrieved from
  9. University of Chicago Medical Center, 280 million-year-old fossil reveals origins of chimaeroid fishes. (2017, January 04).

10. Wilcox, C. (2012, July 28). Evolution: Out Of The Sea. Retrieved from

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