Now that there is a general understanding of the symplesiomorphies of the vertebrates, it will now be easier to identify the characteristics that are unique to each family. To start, we will look at the Myxiniformes, also known as hagfish. Hagfish are unique in that they have remanence of Hemal arches which indicate a linkage to the vertebrates. However, this trait was lost in the species so while it is not actively in hagfish, the presence of the ancestral trait is what links them to the rest of the taxon. Aside from this, Hagfish have no paired fins, a slight caudal tail, a linear brain on a cartilaginous plate (brain case), and a protruding jaw used for tunneling and burrowing into food. Hagfish also have either 5 or 8 gill slits, there is no in between. These characteristics are unique to hagfish and are therefore referred to as apomorphic characteristics.
Next, we will conclude by look at the apomorphic characteristics of Petromysontiformes. This too is a Latin name which can be loosely translated into “stone sucking fish” or as we call them, lampreys. Lampreys are one of the more unique vertebral fishes in that they were the first to develop a lateral line system. This system is used as a sensory mechanism to detect pressure changes in the water which could indicate the presence of a predator or prey. This system is now common in fishes but was first seen in the lamprey. Other characteristics of lampreys include, three eyes (two lateral and image forming, one dorsal), one dorsal nostril, seven gill slits, and a dorsal and tail fin. Like the hagfish, lampreys don’t have any bones or paired fins. However, lampreys do have the ability to survive in both marine and freshwater systems. Fishes with such capabilities are referred to as anadromous fish. Similar to the lateral line system, lampreys were the first to adopt this style of living.
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