Why are some of the world's largest animals vertebrates?
Life on Earth comes in an astonishing number of forms, shapes and sizes. The evolutionary milestone of forming a notochord at least 530 mya marks the beginning of a new lineage. Not only does a notochord provided structural support to some of the earliest invertebrate chordates; it also acted as a site for muscle attachment and is the precursor for vertebral development.
Aside from being a crucial element that forms the strong spine you and I would need to stand upright; the development of a vertebra column also allows vertebrates to grow larger without needing to deal with issues that stemmed from having an exoskeleton. These include periodic vulnerability post-molting and a self-limiting factor that prevented parts of the body from growing too thick and too heavy during growth. According to the Square-cube law, any object that has been doubled in body size would have its mass and volume increased by a factor of eight, whereas its ability to support itself, as in the cross-sectional area, would have only increased by a factor of four. Although this rule also applies to the endoskeleton of vertebrates, a combination of bones, muscles and tendons working together has provided stronger structural integrity than what an exoskeleton alone is capable of offering.
Also known as Metazoa, "Animalia" is a major taxonomic group that includes all animals. Animals are a diverse group of multicellular, eukaryotic (cells containing a nucleus) organisms that are characterized by several key features, including the ability to move, the presence of specialized tissues and organs, and the absence of a cell wall. They are heterotrophic, meaning that they obtain their energy and nutrients by consuming other organisms, and they have a complex nervous system that allows them to respond to stimuli in their environment. Some of the most well-known animal groups include mammals, birds, reptiles, amphibians, and fishes. Together, these five animal classes are grouped under the Chordata phylum by having a notochord (a flexible supporting rod on the back side) some time in their life cycle and other shared features.
Other more recognizable animal groups include insects, crustaceans (e.g., crabs), myriapods (e.g., centipedes), and chelicerates (e.g., spiders, scorpions) which belong to the jointed-appendages phylum Arthropoda, earthworms and leeches which belong to the segmented-worm phylum Annelida, starfish, sea cucumbers, and sea urchins which belong to the spiny-skin phylum Echinodermata, as well as snails, octopuses, and clams which belong to the soft-bodied phylum Mollusca.
It is absolutely imperative to understand the following points:
"Phylum" is a higher classification that is a totally different concept from that of a "species"
Some animal phyla can contain merely one species while some can contain millions (e.g., Arthropoda)
Traditionally grouped under 3 classes, but have since underwent on-going major revisions as phylogenomic and other molecular phylogenetic analyses show errors in grouping based on morphology alone. Some taxa have been excluded to form new phyla, whereas some have been included into Annelida.
Disclaimer: Depending on the reference sources, as well as the techniques and approaches used by systematists to categorize different animal groups, the figures listed for each taxonomic rank could vary drastically. These disparities are especially apparent in larger phyla such as Nematoda, Chordata, Arthropoda, and Brachiopoda. Nonetheless, these data are compiled to the best of my knowledge using latest literature and leading taxonomic databases as of November 2020. If you have access to the latest taxonomic rank updates for specific phyla or would to point out any errors on this page, please do contact me so that I can add you as a contributor. Your input would be much appreciated.