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Why are some of the world's largest animals vertebrates?

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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.

Kingdom: Plantae

Plantae is a classification of living organisms characterized by their ability to produce their own food through the process of photosynthesis (using chlorophyll and other pigments to capture light and later converting it into sugars), with the some exceptions of some mycoheterotrophic species which obtain their nutrients through a symbiotic relationship with soil fungi. Plants play a crucial role in the Earth's ecosystem, providing habitat and food for a wide range of other organisms, as well as producing oxygen and helping to regulate the planet's climate. Examples of plants include mosses, ferns, conifers, and flowering plants. Some of the most important and economically significant plants include crops such as wheat, rice, corn, and soybeans, as well as trees used for lumber, paper production, and other purposes.

Annelida

Kingdom: Fungi

Similar to Animalia and Plantae, Fungi is a taxonomic classification that includes only eukaryotes (organisms that have a nucleus in their cells). However, fungi are composed of branching filaments called hyphae, which form the base of mycelium. All fungi lack chlorophyll and are heterotrophs that obtain their nutrients by absorbing organic matter from living or dead organisms. They are also distinguished from plants by their chitin-containing cell walls. With the exception of the Chytrids, the cells of all other fungi group also lack flagella (whip-like structures for locomotion). This means that most fungi group cannot produce mobile reproductive cells, and that in order to reproduce sexually, their spores must come into direct physical contact with another fungi body. These spores are capable of withstanding extreme cold and lack of water. Some fungi also reproduce asexually by budding. They play important roles in various ecosystems, such as decomposing organic matter, forming mutualistic relationships with plants, and serving as pathogens of plants and animals. Some common examples of fungi include mushrooms, yeasts, molds, and rusts.

Annelida

Kingdom: Animalia

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)

Annelida

1, 2

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.

Subphylum

-None-

Class

Order

Family

Genus

Species

22000

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Arthropoda

Subphylum

Chelicerata, Crustacea, Hexapoda, Myriapoda, †Trilobitomorpha

Class

Order

Family

Genus

Species

1257040

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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.