Amphioxus is a strange little beast. It’s a kind of worm and, in fact, my favorite worm. It is built on a plan that is upside down to most other worms and this gives it a special place in taxonomy because, arguably, it may be our closest invertebrate relative. Why? Well for starters it is a deuterostome, not a protostome. In all multicelled animals, the ball of cells that develops from the zygote is called a blastula and the first hole that appears in that ball of cells is called the blastopore. In most of the animal kingdom, the blastopore becomes the mouth of the organism and these organisms are called protostomes. In one small group though, the blastopore closes or becomes the anus and a separate opening forms the mouth. These are the deuterostomes.
The deuterostomes are not numerically as dominant as the protostomes, which include all the annelids, molluscs and arthropods, plus a number of minor invertebrate phyla. But we are deuterostomes, as are all members of the Phylum Chordata, and what we lack in numbers, we make up for in size. The largest invertebrates are restricted from further size increases by the limitations imposed by exoskeletons in the arthropods and no skeletons in the rest. And it appears it all started with a little upside down worm, a common ancestor we share with the living Amphioxus, whose blastopore did not develop into a mouth.
What other animals are deuterostomes? All the echinoderms share this with us but they went off on their own radially symmetrical path and separated from us a long time ago. There are three other deuterostome groups that share an important chordate feature, the notochord. Amphioxus is one and is put in the Phylum Cephalochordata. The strange sponge-like tunicates are another and they form the phylum Urochordata. Adult tunicates are sessile creatures but as larvae they have a notochord. Lastly are the Hemichordates or acorn worms which also have a notochord.
It is the position of the gut and nerve chord which makes Amphioxus an upside-down worm. In the Protostomes, the gut develops along the dorsal surface. Think of cleaning shrimp. You pull a thin layer of muscle off the back to expose the gut, which can then be stripped away from the animal. And the protostomes have put the nerve chord ventrally. But in Amphioxus and all the chordates, the gut is ventral and the nerve chord dorsal, just above the notochord, upside down from the annelid/arthropod pattern. The notochord itself is a cartilage-like rod that extends the length of the body, giving support to the soft tissues. In higher chordates it is replaced by cartilage and then bone, but this was the start of the internal skeletal system that characterises the higher chordates.
So what is Amphioxus besides an upside down worm? It is laterally compressed and transparent in color and its common name is Lancelet. It lives in the benthos in shallow waters in many places around the world. It is so common in Asia that it is harvested as a food source even though each worm is only a few centimeters long. It has a muscular pharynx surrounded by tentacles and it spends its life, appropriately, upside down in the sand with its tail sticking out for air and its head end gobbling up food items in the sand and mud.
As well as a dorsal notochord and nerve chord, Amphioxus has other primitive chordate characteristics. There are pharyngeal gill openings which the amphioxus uses for filtering food particles but which were adapted for oxygen extraction by the sharks and fish. There are segmental muscles and the beginnings of structures that could be modified later to become fins in fish, the gelatinous fin rays. The amphioxus also has a fish-like tail which it can use for swimming when it is not burrowing in the mud.
There are some features of the lancelet which are not primitive and show that this animal has evolved along its own line for a long time, adapting to its life style. Its feeding apparatus is one example. It has no eyes or well developed sense organs and the nerve chord does not end in a brain. This may not be a primitive condition but related to the lancelet’s sedentary habits.
I like this little worm because, however humble its lifestyle, it shows us where we probably came from, way back in the Cambrian Period – a worm with a difference.
References: Buchsbaum, R. 1968. Animals without Backbones. Penguin Ed.
Parker and Haswell 1964. A text-book of Zoology Vol. 2. Macmillan Student Ed.