About Invertebrates
Invertebrates are, simply put, animals without spines (aka backbones). While this may seem to be only a few types at first glance, it’s actually surprising that over 90% of the total different species of animals in the world are invertebrates. While a majority of the well known and most common species of animals on the planet have backbones, like mammals, birds, and reptiles, quite a few other life forms are born without spines, and have alternative means of sending their neutral signals or physically giving their internal organs a framework.
One of the most basic forms of life, protozoas, have no backbones. These are tiny, single-celled organisms like bacteria, and their microscopic size alone is enough reason for them not to require a spinal column. Taking this form of life and going a step up in size, we’ve got annelids, which are creatures whose bodies are segmented and whose internal organs are efficient enough that you can cut them in half and both halves will grow into a separate annelid. The most well known example of annelids are earthworms.
Aside from having NO spine or bone whatsoever, other invertebrates instead DO have bones in their bodies, but their physiological arrangement is different from vertebrates. Echinoderms are a mainly aquatic example of this. Echinoderms are creatures whose internal organs are arranged at the center of their bodies and, from this center, they grow spiny protrusions that radiate outwards to protect the soft, vulnerable central core. Starfish and sea urchins are examples of this. Echinoderms aren’t built to be highly ambulatory due to the arrangement of their spines.
Other invertebrates in the ocean include mollusks, who similarly have a soft central core but, instead of protecting it with spiny outgrowths like echinoderms, they rely instead on a dense and heavy outer armored shell. Clams and oysters are the most common example of this, though mollusks aren’t restricted to seagoing life forms. The snail is likewise a form of mollusk. Mollusks are a bit more mobile than echinoderms, often employing one organ dedicated to propulsion. The snail has a single foot which secretes fluids on which it slides forward, and clams have small tubes they can stick out of their shells to squirt out water in a tiny jet that pushes them around.
The ocean seems to hold the greatest number of invertebrates, as there are also SOFT skinned mollusks like squid and octopus. Given the fluid nature of water, soft, shell-less mollusks give up the protection offered by their hard-shelled versions in favor of sheer mobility. They actually have appendages, often quite a large number of them, and their musculature is much stronger than hard shelled mollusks yet designed for full flexibility to take advantage of their underwater habitat.
Shells, however, are a common defining factor for another type of underwater invertebrate, the crustacean. The biggest difference between a mollusk and a crustacean is the lack of a central core. Crustaceans actually have a main body which houses conventional internal organs like the heart and stomach, they actually have a head, and even appendages like legs and claws. Crustaceans are probably one of the best known form of invertebrate, as most of them wind up on a lunch or dinner plate; aka crabs, shrimps, and lobster. Oh yum….
Finally, moving out of the water, we get to the MAIN reason why invertebrates actually outnumber vertebrates on the planet. Insects. Bugs. Spiders. Creepy, crawly little things… call them what you want, but insects are actually one of the most hardy and efficient life forms on the planet in terms of sheer physical power and survivability. Insects get by without a spine by instead forming hard exoskeletons which cover their entire bodies. Their bodies are made up of a head, a thorax which houses most of the internal organs, and an abdomen. Their appendages are likewise covered by their exoskeletons. The muscle arrangement of insects within their exoskeletons actually gives them some of the highest strength to body weight ratio of any species of animal on the planet. Similarly, their internal organs are denser, and function so effectively that some toxins and even radiation don’t affect them as much as vertebrates, so that it’s been held that, in the event of something like a nuclear holocaust, they actually have the highest chances of survival.
Tags: animals, invertebrates
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