Many years ago, I visited the Dehiwela Zoo. This is a beautiful zoo, that even has a lake where pelicans and other birds visit. Naturally, the exhibits featured only vertebrates—animals with backbones and skeletons. Even with my limited knowledge at the time, I knew there existed another vast world of creatures: invertebrates, some so strange that they seemed almost alien. One day, curiosity got the better of me, and I decided to find out more.
All animals are classified into about 35 major groups known as phyla, though only around nine are widely familiar. All vertebrates—mammals, birds, fish, reptiles, and amphibians—belong to a single phylum: Chordata. That realization alone is humbling. But what about the rest?
Take phylum Mollusca, for instance. A snail may appear unremarkable at first glance, but its body plan is distinctive: a muscular foot, a mantle that often produces a shell, and surprisingly complex eyes. Like many invertebrates, snails lack a spinal cord or a single centralized brain. Instead, they possess clusters of neurons called ganglia, each responsible for different bodily functions. Slugs, close relatives of snails, seem to live by a simple rule—“just hang on”—constantly battling dehydration, predators, and the elements.
It comes as a surprise, then, that one of the most intelligent animals on Earth—the octopus—is also a mollusk. I once read a strangely convincing account by a renowned science fiction writer suggesting that octopuses might have developed advanced civilizations had they not been confined to the ocean, unable to harness fire or forge tools. Speculative though it may be, the idea lingers. And then there is the giant squid: elusive, haunting, and seemingly drawn straight from a nightmare. Mollusca is, in fact, the second most diverse animal phylum after Arthropoda, comprising roughly 93,000 species.
For a long time, I had thought starfish rather overrated in literature, particularly poetry. That opinion changed one day while wading in the shallow, crystal-clear waters of Trincomalee. I spotted a large orange starfish resting on the seabed. As I moved closer, it glided effortlessly away. The motion was silent, fluid, and utterly magical—unlike anything I had seen before. Starfish, despite their name, are not fish at all but belong to the phylum Echinodermata, along with sea urchins, sea cucumbers, and sand dollars. All members of this group are found exclusively in the sea.
Not all invertebrates inspire such awe. Some are repulsive, some deadly, and others so simple they could easily be mistaken for plants. Consider the sponge, from the phylum Porifera—the simplest of all animals. Its motto might well be “I ain’t going nowhere,” as it remains permanently attached to the seabed. Sponges are multicellular and heterotrophic, lacking cell walls yet they possess no true tissues or organs. Their bodies are shaped for one purpose alone: to allow water to flow efficiently through a central cavity, delivering nutrients before exiting through an opening. And that, remarkably, is enough.
If invertebrates ever seem dull, insects quickly put that notion to rest. Insects belong to Arthropoda, the most diverse animal phylum of all. This vast group also includes centipedes and millipedes, spiders and scorpions, prawns and crabs. At first glance, it is hard to imagine an insect and a crab as relatives—but look closely at their jointed limbs and segmented bodies, and the family resemblance begins to emerge.
That, perhaps, is the quiet power of classification. It reveals hidden connections beneath surface appearances, reminding us that the living world is far stranger, richer, and more interconnected than it first appears. What once seemed alien or insignificant becomes, with understanding, deeply familiar—and endlessly fascinating.
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