The last session in my dissection course is a two-part dissection of a pig and a frog. This one goes into much greater detail than any other dissection in the course, so there’s rarely time for a protracted video presentation at the end. Still, I keep a good selection of surprises for my students.
As my dissection course draws to a close, it naturally begins moving toward the clade of animals most familiar to my students: the chordates. On its way there, it visits the chordates’ closet living relatives, the echinoderms. Students usually need some convincing that echinoderms and chordates and echinoderms have anything in common, because the adult forms do indeed have no real similarities. The evidence of the kinship between these groups, and between the vertebrates and the other chordates, is mostly genetic and embryonic. These highly divergent animals have a number of highly improbable similarities in the way their embryos form and the anatomy of their larvae, revealing that their adult forms are highly specialized rather than ancestrally distinct.
I usually start with the primitive chordates, because they’re somewhat less spectacular in shape than the echinoderms. The students get to see whole lancelets as well as numerous sections through them, so this sister group to the vertebrates is well known to them. The other non-vertebrate chordate group is the highly underrated Urochordata. These animals have all of the classic chordate characters as larvae but lose almost all of them as adults. All urochordates are filter feeders, but they have highly dissimilar means of realizing that lifestyle.
And then we arrive at the insects.
My university has a very limited array of specialized high-level zoology courses. There are no high-level electives about the intricacies of lampshells or even mollusks at the University of Ottawa. There are courses about various groups of vertebrates, and about unusual microbes like the microsporidia that get a lot of attention for other reasons. We do have an entomology course, because insects are that fundamental and that numerous.
Humans talk a mean game about being the dominant organism on planet Earth, and it’s not an unreasonable assertion. Humans are on a short list of species found on all continents (no matter how those continents are parsed) and most of the other contenders are animals like cattle whose ecology is intimately entangled with ours. If we compare humans (or even primates at large, to be honest) with insects, though…there are millions of them for every one of us, and they are omnipresent. Insects are defining features of every ecology except for the oceans, and a few visit even there. As noted earlier, the insects have more species to their name than any other taxonomically similar group of organisms.
So it’s no surprise that some very interesting beasties lurk in this massive assemblage.
The chelicerates are likely the most primitive of the extant arthropod groups, and they are the simplest anatomically. Chelicerates are one of the first groups of animals known to have made the move from water to land, but their dominion over terrestrial ecosystems has not lasted. Nowadays, the spaces that once belonged to this ancient lineage mostly belong to crustaceans in water and insects (so, different crustaceans) on land. Still, chelicerates remain a major ecological force, thanks to the multitudinous mites and ticks and the prevalence of spiders and scorpions as insect predators.
And, they are magnificently weird.
The arthropods are the only phylum covered in my course that is split over multiple sessions, and with good reason. Arthropods are a majority of the species known to science. Parse that carefully: not a majority of invertebrate species. Not a majority of animal species. A majority of species, period.
That massive diversity means that arthropods are also impressively different from one another across their various groups, and covering that diversity requires an extended fraction of the lecture course’s time and two entire lab sessions. Which groups are covered in which sessions changes from year to year, so for this series I’m doing the three “classical” arthropod subphyla–crustaceans, chelicerates, and “atelocerates”–each as their own post. Today’s topic is crustaceans.
After the tour of seashore beauties in the previous chapter, the annelids and nematodes may seem dull. The very limited array of annelids that most people encounter–mostly just the one–certainly don’t help that impression, and nematodes all look more-or-less alike. Like any other animal phylum, though, the annelids have a few surprises for us.
The next big chapter in the course I teach is on the mollusks. The most visible component of any seashore’s biota, mollusks are an incredibly diverse group of animals. One could be forgiven for not knowing in advance that snails, clams, and the colossal squid are about as close together on the tree of life as spiders and lobsters, or humans and pipefish. Mollusks are, in this way, a classic example of adaptive radiation, in which an ancestral body plan somewhat like a very primitive snail was reshaped into widely dissimilar beasts in response to very different selection pressures.