The darkness is everywhere in this pitch black and humid forest. Unaware of the ancient hunter that is slowly wiggling its way through the undergrowth on its cute stubby legs, you are cleaning yourself after a long and tiring day. Suddenly, you’re stuck in a mass of glue and are no longer able to move. Only now do you see the hunter, whose fangs glitter in the moonlight. You’re a cricket about to succumb to the fearsome velvet worm. Watch this amazing scene unfold below, narrated by David Attenborough himself.
The head of velvet worm looks like it came straight from a 50s monster movie, with its huge antenna and gaping jaws. You might mistake the little globes below the antenna for eyes, but they’re actually the slime papillae that the velvet worm uses for spitting its sticky glue. Believe it or not, the structure, function and origins of this monstrous head have been the object of intense debate for more than a century. I guess that’s the price to pay if embryologists, anatomists, palaeontologists, and molecular biologists each bring their own insights to the table.
Like in other segmented creatures such as arthropods, the head of velvet worms is not clearly separated from the rest of its body. Instead, the antenna, jaws and slime papillae seem to be modified appendages of three different segments. Since arthropods and velvet worms are closely related, you’d expect some of the segments to be homologous. Comparing and aligning the segments of velvet worms and arthropods is surprisingly troublesome though. The antenna of velvet worms are placed on the first segment, whereas arthropods bear their first antenna on their second body segment. Continuing the confusion, velvet worms have their jaws on the second segment while arthropods carry their mandibles and maxilla on the third and fourth segments.
Despite these discrepancies, arthropod and velvet worm heads have often been aligned based on their segments, like in the figure above. Since arthropods have their brains spread out over their three first segments, many people assumed that velvet worms have a similar organization of their brains. However, new research by Mayer and colleagues challenges this assumption and shows that segmentation might not always be the right guideline for interpreting the origin of heads.
Like tracing back an extension cord to see where it is plugged in, the scientists traced back the origins of the nerves of the antenna, jaws and slime papillae. Sure enough, the nerves innervating the antenna and jaws were ‘plugged’ into two distinct regions of the brain. But the nerves leading to the slime papillae come directly from the nerve cord instead, instead of from a dedicated part of the brain. Additionally, they show that what was previously perceived as a tripartite brain in adult velvet worms, actually originates from a single centre in the velvet worm embryo.
These findings suggest that velvet worms do not posses a tripartite brain like arthropods do. The number of ‘brainy segments’ likely increased in the arthropod lineage, while the velvet worms stayed bipartite. Coincidentally, the brains of vertebrates also consists of three different types of tissue. Based on this observation and the conserved gene expression of developmental genes between arthropods and vertebrates, it has sometimes been suggested that a tripartite brain must have been one of the features of the ‘urbilateral‘ ancestor of all bilateral animals. This research shows that our great-great ancestor was unlikely to have three different brain parts.
She seemed to get along just fine though, becoming the ancestor of the whole bizarre range of bilateral animals. Velvet worms included.
Mayer G, Whitington PM, Sunnucks P, & Pflueger HJ (2010). A revision of brain composition in Onychophora (velvet worms) suggests that the tritocerebrum evolved in arthropods. BMC evolutionary biology, 10 (1) PMID: 20727203
You might also like: