A Amphioxus/Branchiostoma is a primitive chordate that would probably look to most non-biologists like a tiny fish or even a tadpole. Thing is, amphioxuses aren’t fish, they’re not even vertebrates!
In fact, amphioxus belongs to a sister subphylum, “cephalochordata” (any pharyngulites who might be reading this will reconize “cephalo” from “cephalopod”. This is because “cephalo” means “head” and apparently, octopussies walk on their heads🙂 ).
Both vertebrates and cephalochordates belong to the phylum Chordata – which began it’s history pretty much with most of modern-day phyla, somewhere in the cambrian explosion.
The amphioxus shows some amazing qualities that make it an interesting animal to know about, and also an interesting model species for comparative anatomy and evolutionary research.
The reason for this being that the amphioxus presents a lot of qualities that make him a living transitional fossil, or simply, a transitional species.
First of all, this critter gives all the indication for being a primitive invertebrate with distinct differences from the more modern taxa.
Second, it shows characteristics that are peculiar to the amphioxus, but are also highly indicative of “later”, evolved traits.
Third, it shows characteristics that although primitive, are still extant in variations today.
Let’s start with the primitive stuff. The amphioxus has no vascular system, that is, he doesn’t breathe! The amphioxus absorbs his oxygen via diffusion, which is an extremely poor method of gaining oxygen, and at any case, results in much less oxygen absorption. The reason for this is that amphioxus has a very small body with a very relatively large surface area. This means that by diffusing the oxygen, the amphioxus gets just what he needs.
Also, the amphioxus has no real brain or “brain concentration” – he does have a neural system – which belongs more to my second category.
The amphioxus also has no eyes, and he spends most of his time buried in the sand, filtering food (he doesn’t have teeth, of course, while not having a backbone as well).
Things start to get interesting when you look at traits in amphioxus that are uncannily “vertebrate-like”. For starters, the amphioxus has a precursor for the liver that doesn’t function like one. It has similar hepatic characteristics, yet a stubbornly “non-hepatic” function in the amphioxus. This is the “hepatic diverticulum”, and I’ll leave it at that for anyone nerdy enough to get deeper into it.
Also, amphioxuses have dorsal segmental muscles, much like the muscles we have between our vertebrae in our back.
The most amazing feautures about the amphioxus are the features which shed light on chordate evolution. For starters, amphioxuses have glands very much like vertebrates do, which is funny, since if you guys were paying any attention, you’d remind me that I just said that he has no vascular system. Since glands are used to secrete hormones, which are signal molecules transmitted through the blood, it’s kinda hard having signal molecules sent through blood when there isn’t any!
The amphioxus glands are fascinating because of their location and their mode of operation. For example. There is a structure called Hatschek’s pit exactly where the hypothalamus should be. It has a “hypophysis” or pituitary gland-like structure called “Rathke’s pouch” in a similar location and similar structure, that produces “exohormones”! This primitive gland secretes external signal molecules, and, according to one zoology professor I had, these molecules are similar in function and chemical activity to the hormones modern vertebrates secrete from their evolved pituitary glands!
So, really, what we have here is not just another squiggly tadpole, but a precursor to modern vertebrates, with genes, anatomy and physiology to tell us the story of how we evolved.