Lobsters and pill bugs are crustaceans, and even in the same subclass. They and insects are all part of Phylum Arthropoda, though.
Like this:
Phylum Arthropoda
>Class Insecta
>several other classes not relevant to this discussion, like Arachnida and Myriapoda
>Class Crustacea
>>numerous subclasses of small nektonic creatures, plus Cirripedia, the barnacles
>>Subclass Malacostraca. “Typically with compound eyes, stalked or sessile; a carapace covering the cephalothorax; 8 thoracic and 6 adbdominal somites, each with paired appendages.”
>>>scads of primitive shrimplike forms in various superorders
>>>Superorder Peracarida (description on request)
>>>>numerous shrimplike creatures and beachhoppers
>>>>Order Isopoda, the pill bugs
>>>Superorder Eucarida
>>>>Order Euphausiacea, the krill
>>>>Order Decapoda, the shrimps, lobsters, and crabs
>>>>>Suborder Natantia, the shrimps
>>>>>Suborder Reptantia (description on request)
>>>>>>Section Macrura (description on request)
>>>>>>>Superfamily Nephropidea (lobsters and crayfish)
>>>>>>>>Family Nephropidae, the lobsters
BTW, to cover something else brought up by the OP, not explicitly mentioned here - Lobsters, at least the common supermarket variety, have 10 legs, counting the front ones modified as claws. Hence the order “Decapoda”, as noted by Polycarp.
A somewhat apt analogy, since lobsters are approximately as closely related to insects as chickens are to tuna. In both cases, the animals are in the same phylum (arthropods for the insects and lobster, vertibrates for the tuna and chicken) but different classes (crustacean for the lobster vs. insect, and fish vs. bird). Doubtless one of our resident taxonomists will expand on this: I’m not sure exactly how “closely related” is implied by the same or different phylum or class, and I may have not quite drawn the right lines between the taxa (I can never remember whether “vertibrate” or “chordate” is the phylum, for instance).
Meanwhile, I vaguely recall hearing of an invertebrate critter which had an exoskeleton covering jointed legs, but not the rest of its body. It vaguely resembled a cross between a centipede and a worm. What was that thing called, and how is it classified nowadays? Is it an arthropod or an annelid, or something else altogether, and is it related to the putative transitional form between annelids and arthropods?
Of course, non-biologic classification of organisms can lead to different results. In one of Richard Dawkins’ books, he mentions a court case concerning whether boiling lobsters live fell under a “cruelty to animals” statute. The court decision was that it did not because “Lobsters are insects, not animals”, a rather neat way of making at least two taxonomically wrong statements in 5 words.
This is why it is my firm belief that Linnaean taxonomy is crap. It was developed simply to group “like with like”, and as such provides little information regarding actual relationships (simply look at the vertebrate classes of Reptilia, Aves and Mammalia; Aves descended from Reptilia, while Reptilia and Mammalia share a common ancestor). Although, for the record, the Phylum for vertebrates is Chordata, while Vertebrata itself is a subphylum. Blech. Now I feel all dirty.
With respect to our lobster, insect and isopod friends here, all three are members of the clade Arthropoda. Within Arthropoda, insects are members of Hexapoda, while isopods and lobsters are in the clade Crustacea. Hexapoda and Crustacea are sister groups within Arthropoda, meaning that overall, the groups are fairly closesly related (in other words, they share a common ancestor).
Meanwhile, both isopods and lobsters belong to the clade Malacostraca (which also includes mantis shrimps, krill, and “sea fleas”) within Crustacea. There are a couple more subdivisions within Malacostraca before we get to Caradoidia, within which we still have isopods and lobsters. Caradoidia is usually broken into four branches. One branch leads to the isopods (Peracarida), while another leads to Decapoda (lobsters, crabs, shrimp, etc.) + Amphionides (a planktonic criter; note that Amphionides is a genus, while Dexcapoda is a Linnaean Order).
So, isopods and lobsters are somewhat closely related, while lobsters and bugs and quite distantly related.
You’re probably thinking of phylum Onychophora, whose members look sorta like caterpillars, except that that is their mature form. Here’s a close-up (and there are lots more images where this came from:
Onychophora is also thought to be a sister group to Arthropoda (or a subphylum thereof, depending on who you ask). The (in)famous Cambrian-era Hallucigenia is now often classified as an Onychophoran.
I remember watching some discovery channel type documentary and they found some crustacean that looked like a giant woodlouse about a foot long or something maybe a little bigger or smaller and it lived deep underwater…I think they found it around the Gulf of Mexico…does anyone else remember this?
Here’s(warning! shoddy pseudoscience) the link that came up a week or so ago in an evolution thread.
[Sidetrack]I visited a mineral/fossil shop today and they had on sale the most exquisite trilobite fossils - they must somehow have been preserved without being crushed at all (perhaps in really soft, fine silt) and the fossils had been exposed from the underneath - the legs and mouthparts of the creature extending free, right out of the rock - otherworldly and fascinating.
I remember seeing that thing on the nature channel. It was too big to fit into the sub’s sample tube so when it rolled up they just sucked the tube onto it’s back and dropped it into the specimin basket. I remember thinking how hillarious it would be to sneak one of those into someone’s garden and wait for the scream.
Ummm, what would you replace it with? Isn’t every split going to be into two or maybe three parts? With KPCOFGS and maybe a couple of sub-whatevers thrown in, you’ve got on the order of ten levels. If you put in a new level every time the, say, human branch split from its common ancestor, wouldn’t you have something like a hundred levels?
Starting from the earliest life, we (eukaryotes) split from prokaryotes, then we split from archaebacteria, then we split from protists, then from plants, then from fungi, then from worms, then the vertabrates split from invertabrates. Seven splits, and we’re only down to phylum. To follow the actual splits every time would seem to be too many divisions to be helpful.
The answer is cladistics, which I’ll let Darwin’s Finch explain authoritatively and defend. But the logic in this is that by defining clades, not as specific hierarchical levels in the Linnaean model, you have the flexibility to make the subsidiary units you want. For example, rodents and pangolins both constitute orders of mammals – but getting a coherent breakdown of the relative closeness and distance of interrelationships among rodents is far more complex than is the case for the handful of species of pangolins. So by allowing multiple clades tiered without the need for new coinages of “infracohorts” and “supertribes” to do that sort of breakdown, you’re afforded that flexibility.
The problem with cladistics is that it depends irrecovably on choosing the right bifurcations in the right order. This can very easily be illustrated by the recent to-do about man, chimpanzees, and bonobos – where the cladistic school of thought made a reasonable assumption about DNA divergence that was later proved to be incorrect.
As Polycarp mentions, cladistics is the prevailing method of taxonomy these days (or, at least, rapidly becoming so). You certainly could (and, indeed, do) wind up with many, many levels, but not all of these nodes require a name. Only those that are evolutionarily meaningful tend to be given formal names, and many nodes remain unnamed (the node uniting Hexapoda and Crustacea, for example, is unnamed, as is the Decapoda + Amphionides node).
There are two key benefits to using cladistics: first, one needn’t concern oneself with that actual name of the hierarchical level, in addition to the name of the level itself (it doesn’t matter whether Crustacea is a phylum or family or order or genus or anything else), and second, it preserves evolutionary relationships. The latter is the primary reason for preferring cladistics, in my opinion. Regardless of how many nodes one has, one can still determine who is related to whom based on the branching nodes. We can tell that Crustaceans and Arthropods are only distantly related, while Isopods and Crustaceans are closely related. Using Linnaean taxonomy, we can see what is morphologically similar, but we don’t get anything close to the evolutionary insight provided by cladistic methods.
Another big concern is that anything higher than the species level in Linnaean taxonomy is completely arbitrary to begin with. ALL groups in cladistics are real, whether named or not (at least, the goal is to create only real groups). By “real”, I mean that group A descended from group B, and is therefore a subset of group B. We are included within Vertebrata because we are ultimately descendants of the first vertebrates. Even if a group later loses certain features, evolutionary relationships are maintained; whales remain Tetrapods despite having no feet, and birds remain dinosaurs, and, by extension, reptiles as well.
Some folks would like it both ways. As a result, it is not uncommon to find far more than the 10 subdivisions you describe. There are infra-, supra-, sub-, and super-groups. Then you get Tribes and Cohorts and a bunch of other “new” levels to contend with. To get from Kingdom to species for a humans, for example, can require up to 14 subdivisions nowadays, including subphylum, superclass, subclass, infraclass, superorder, infraorder, and superfamily, in addtion to the “big 7”. Some groups have even more! While none of these subgroups are “mandatory”, they are still often used - Vertebrata, for example, is a subphylum, which would be lost if we just stuck to the standard 7 levels.
