Phylum Cycliophora
protonephridia
periodically
form and a feeding form
and a sexually-produced chordoid larvae
Cycliophora relations:
18S Ribosomal RNA genes suggest Rotifers and acanthocephalans
Ciliated ring reminescent of rotifers
Dwarf males present
Some rotifers are parasitic on crustacea
Cuticle suggests a pseudocoelomate association
Protonephridia are found in many unrelated phyla
U-shaped digestive system doesn’t resemble that of acanthocephalans or rotifers.
Unclear what they are related to.
Xenoturbella bocki
A marine worm was first described in 1949 as an acoel flatworm.
Later thought to be a deuterostome
Gamete development data and 18S rRna and mitochondrial genes also suggests affinities to bivalve molluscs
A second species was discovered last year.
4 cm long. Vermiform, Covered by locomotory cilia
No digestive tract
No organs at all.
A statocysts is present but no other appendages apparent on the outside of the body.
20-100m sediments found.
Off the coast of Scotland and Sweden.
Spiral cleavage present
Trochophore larvae seen in the more advanced developmental stages
Conspicuous mouth, gills, and digestive tract that then degenerates later.
Named after Sixten Bock (1884-1946) who is a famous polyclade turbellarian specialist (Swedish). He first found the worm and put it in a jar and thought it was an acoel flatworm.
Has radial, circular, and longitudinal muscles present.
Opens a mouth pore and protrudes a ciliated gastrodermis to feed.
Moves very slowly with cilia.
Has a trochophore larval stage and spiral cleavage, but seems to have indeterminate cell fate.
Phylum Hemichordata
enterocoely or schizocoely
(protocoel, mesocoel, metacoel)
3) Ciliated pharyngeal gill slits
3) Open circulatory system
4) Glomerulus excretory structures
5) Complete gut-may be U-shaped
6) Sometimes hollow nerve cord
7) Dioecious, but asexual reproduction common-solitary or colonial
8) Body divided into three regions:
preoral lobe (proboscis), collar, and
trunk with corresponding coelomic
divisions
9) Marine benthic
Hemichordates are a group of benthic dwelling deposit feeding or filter-feeding animals that are unremarkable in most regards. They are interesting to us primarily for one reason-they tell us something about the evolution of ourselves since most phylogenetic trees show this phyla as our closest relatives.
There are two major classes of hemichordates that differ quite a bit (which may eventually prove to separate the two into different phyla?)
Pterobranchia-hydroid-like colonies of tiny zooids (21 species)
Enteropneusta-large solitary worms (70 species)
1) They are coelomate deuterostomes-although the coelom may form by enterocoely or schizocoely depending on the class. Cleavage is radial-at least initially.-coelom is tripartite like that of the lophophorates and the echinoderms.
2) They have pharyngeal gill slits that function in breathing. The gill slits have stiffening supports that are found between them. They may function in removing excess water from the food-and then took on an additional respiratory function second. The supports around the gills also bear blood vessels and are likely involved in gas exchange
3) The circulatory system is an open one but it has a contractile heart for pushing blood through it. The heart is located in the proboscis and has evaginations that increases surface area as blood passes through. There are podocytes along these evaginations that ultrafilter the blood to the protocoelom-the portion of the coelom lying in the probocis. Urine is formed and drained out of a pore.
This structure is called a glomerus and is a bizarrely unique means of excretion.
Thus the hemichordate heart is also a urine-forming kidney. Now if poets that write love poems could get a hold of that metaphor-wouldn’t that be interesting.
Most of the other facts that make up this phylum are uninteresting-but they have pharyngeal gill slits and a dorsal nerve cord that is sometimes hollow. These are two traits that are shared with the phylum to which we belong.
What are the other two characteristics or synapomorphies that make up our phylum
1) post-anal tail
2) notochord, a stiff set of stacked disc shaped cells with a fibrous matrix surrounding them.
Hemichordates have a stiff structure in the collar called a proboscis skeleton that they use to anchor the heart and muscles in the proboscis. This was thought to be a notochord at one time and hemichordates were considered chordates for this reason. Since they have only half the characteristics of chordates-they are called hemichordates.
Class Enteropneusta (acorn worms)
Most 9-45 cm in length
The common species, Balanoglossus aurantiacus off the southeast coast of the US may be 1 meter
The largest species known can attain lengths of 2 and half meters and live in 3 meter long burrows.
The Ballad of the Hemichordate
I once held a most exhalted post,
Among the animals few,
I had been viewed for years to date,
Nothing less than a complete chordate
I had gill slits, a hollow nerve cord, and notochord to boot
Though my anus was terminal not subterminal, the taxonomists failed to refute-
My chordate classification was assured it would seem
Then morphologists and embryologists began to assemble their respective teams,
And found that my dear notochord was nothing less than a fanciful dream.
The taxonomists cried foul and heaved a collective sigh
When they discovered that the notochord was nothing more than diverticuli!
The name sweet notochord would no longer do. So scurrying with books of jargon, they came up with another name to spew.
It is half stomach and half notochord-the name seems fairly obvious. We should call it a Stomochord to confuse the budding invertebrate zoologist.
We'll cast it out of the special group chordata and lump it with the rest.
It is half the chordate it used to be, a mere hemichordate at best.
And thus another phylum was erected for this evolutionary pest.
And so is this the end of this wretched poem making it quite terse? You wish it were, I'm afraid to say, the poet will spout more verse.
Hemichordates have other virtues than their chordate affinities.
Bestowed upon them millenia past-but not by any divinities.
Poets often write of the heart-its rich symbolic meaning,
Yet if they knew of hemichordate hearts, they would find them more demeaning.
For hemichordate hearts you see are more unique than the shroud of turin-
For along with their ability to pump-they are able to create a dilute urine.
The chordate heart is waxed in verse by poet laureates diverse
But the hemichordate heart is a better token of modern love, no truer words were spoken.
For here it gives life sustaining beats and performs a second metabolic feat.
Its little glomerulus and coelom true packed right next to a vessel for blood to pass through.
So when you hear the music being sung with the words "urine my heart", urine my soul"
Remember that no better description of the hemichordate has been told.