Here’s a couple of strange and obscure groups of invertebrates that most people haven’t heard of: the Priapulida, Kinorhyncha, and Loricifera, some small marine worms that have no medical significance, aren’t used as research models, tend to live in odd and inaccessible places, and each phylum has no more than a few hundred species. They are obscure enough and so little studied that their classification is even in flux—the Loricifera were only recognized as a discrete phylum in 1983, and there is talk of reorganizing all four (including the Nematomorpha) into a single phylum. You’ll also see the groups referred to by different names; the Cycloneuralia, the Introverta, the Scalidophora. They’ve been grouped with arthropods, onychophorans, and tardigrades in the Ecdysozoa.

They’ve been called the Cycloneuralia because they don’t have much of a brain, but what they do have is organized in a ring wrapped around their esophagus. They belong to the Introverta because they have that curiously eversible anterior portion of the head—they can pop their mouth parts in and out on a stalk, as you can see in the figure to the right. The Ecdysozoa are animals that periodically molt their cuticles. They’ve been labeled the Scalidophora because they posess rings of sharp bristles around their mouth parts, called scalids.

Basically, these are weird, spiky, tiny little marine worms that you may never encounter in your life. What’s so interesting about them?

One fascinating thing is that they may be closely related to the arthropods (Aguinalda et al., 1997), which you certainly will encounter, and represent a simpler version of the most diverse and populous animal phylum on the planet.

Another is a recent paper by Dong et al. (2004)—we now have fossils of their embryos from the Cambrian. This is amazing stuff. Embryos of marine invertebrates tend to be tiny, delicate things that are easily destroyed, so no one expected we’d ever find any that were half a billion years old...but there they are. Some of the Cambrian fossil beds in China are unique in composition, formed under unusual chemical conditions, and the details of these tiny specimens are preserved in fine-grained calcium phosphate. Here, for example, is one of the new fossils (Markuelia hunanensis), a tiny blastula less than 250 μm across and made up of less than 500 cells:

Late cleavage embryo of Markuelia with surface margins of blastomeres preserved.

The authors have multiple specimens from this collection, and here’s another, later stage. The embryo is wormlike, curled into a ball around its yolk, and you can see the annulae, or rings, that form its body.

Annulated embryo of Markuelia with yolk tissue.

In yet older animals, we can see the structures that allow the authors to identify the organism. In this close-up of the oral end of the embryo, you can see the array of spines that ring the terminal mouth: it’s got scalids. It belongs to the Scalidophora, the Cycloneuralians.

View of the oral region of Markuelia embryo. (s): scalid.

Another interesting property of these embryos is that they exhibit direct development—that is, they develop into the adult form without taking any detours into a specialized larval feeding morphology. The accompanying News and Views article mentions the curious fact that the extant scalidophorans are so obscure and so poorly studied that this may actually be a case where we have a better picture of the development of a species that died out half a billion years ago than we do of its still-living cousins!

Aguinaldo AMA, Turbeville JM, Linford LS, Rivera MC, Garey JR, Raff RA, Lake JA (1997) Evidence for a clade of nematodes, arthropods and other moulting animals. Nature 387:489-493.

Dong X-p, Donoguhe PCJ, Cheng H, Liu J-b (2004) Fossil embryos from the Middle and Late Cambrian period of Hunan, south China. Nature 427:237-240.